If you're like most of us who have grown up with pop music culture, you've probably owned one better-than-average sound system. Maybe it's grown more and more average of late, and now you're looking for something more sonically sophisticated.

There are several stereo options open to you. First, you can see if repairs will improve your present system. If the answer is no, and it's a whole new system you seek, be prepared to spend $600 to $750 for a truly contemporary stereo. If you're relatively satisfied with your older brother's 1959 console, or if that plastic record player with the fold-up, pull-off extension speakers still thrills you with its convenience, or if you don't want to spend more than a couple hundred bucks, then perhaps you should stick with one of the better "compact" stereos or an inexpensive component setup and read no further.

There's yet another method of improvement: trading up, or selling some or all of your present components to help defray the cost of their better replacements. First of all, ask yourself whether you're the hardcore audiophile who demands near-perfection from listening gear, or someone who just wants Jackson Browne moaning faintly in the background. Then, try to get the best trade-in deal on your old components for the money you want to spend. THE STEREO PHYSICAL

The resale value of your present system hinges on several variables: the brand names involved, the age of the equipment, its workability and external appearance -- even the mood of the appraiser where you hope to exchange the gear. If you have the patience to lug the old components around all Saturday afternoon, try getting a few written trade-in estimates. Generally speaking, it doesn't pay to have faulty equipment repaired before trade-in. Some dealers won't even test your old components prior to quoting you a price. Why plow money into something just to get rid of it at rock-bottom prices?

Before you rush to the hi-fi store, you should be aware that you may not have to dump any part of your present system to achieve significant improvement in sound quality. Investing $20 to $50 in a new stylus (needle) or cartridge assembly can make all the difference in the world, especially since cartridge engineering has advanced markedly in the last five or ten years.

The basic rule is that a good arm and cartridge should apply no more than one and a half grams of pressure to the grooves as it rides the record. The best apply no more than a gram. (A heavier cartridge is bad for both sound quality and the disc).

Another simple, yet inexpensive upgrade is basic servicing. Turntables especially require periodic tune-ups. If your needle sticks on perfectly good records, or if you hear excess noise from one of your speakers, your tone arm may not be pivoting properly and the stylus could be eroding one side of the disc grooves. Workshop maintenance and calibration could help.

Trying to fix more serious troubles could prove futile. Unless your hi-fi gear is still under warranty, buying new equipment sometimes makes better sense than mending an outdated component. This is especially true for damaged turntables; receivers and speakers should be treated on a case-by-case basis. Whatever, always get cost estimates before agreeing to any repairs on your unit. If you can trade up to a new one for under $100 more, consider doing so. COMPONENT UPGRADES

In the absence of an out-and-out breakdown, there are two theories on where to begin your stereo upgrade -- assuming for the moment that you can't immediately swap for a whole new rig. Some experts suggest starting with the turntable, least expensive of the three basic stereo components (and perhaps least important), but a machine that has evolved considerably during the last half-decade.

With its tangled assembly of moving parts and pulleys, not to mention greater exposure to human abuse, yesterday's turntable was more prone to deterioration than a stereo receiver or speakers. Consequently, the trend today is toward simplicity of engineering (fewer working parts) and operation. How many serious music buffs have you see stacking five or six records, punching the automatic play button, and headind for the Ping-Pong room? Few or none. That's because most audiophiles now favor single-play or manual turntables over the old record-changers, and they're content to spin just one platter at a time, a practice that has helped records last longer. If you now own a budget-priced record-changer (one you bought for under $100), you can make a significant improvement by opting for one of the newer belt or direct-drive turntables in the $150 range, and you can have your choice of single- or multiple-play features.

Loudspeakers represent the other method of upgrading, and probably offer the greatest potential for improving the sound of an existing system. Your turntable and receiver -- the latter is often reffered to as a tuner or amp, for amplifier -- deal exclusively with electronic signals, but this energy is translated into pure (one hopes) sound by the speakers. Very good loudspeakers come in all shapes and sizes, tones and "colors" (emphasizing bass and treble, for instance), and in price ranges from under $100 up into the thousands.

Again, your musical tastes should determine the speakers you want. A good speaker will reproduce all sounds well; but if you like classical works, for instance, you might want speakers you can adjust to pick up the sounds of strings; rock fans might want to be able to hear a heavy bass. Bring your own albums to the store to try out on speakers; if you can, get a home trial. There's no substitute for listening to the speakers yourself.

Don't skimp on a pair or good loudspeakers because quality speakers can often make an otherwise mediocre system sound better. For an impressive second stereo system, loudspeakers selling at $125 to $200 apiece will deliver more than enough sonic goods for most homes and apartments.

A more advanced receiver can lessen the undesirable noise that has been inherent in electronic circuitry, heighten the clarity of the highs and lows, and provide convenience features (tuning knobs, input jacks and noise filters) absent from your previous power source. Respected receivers exist for under $300.

But if you've decided to upgrade your listening rig one component at a time, the single most important factor is compatibility. In general, your equipment should be of consistent caliber throughout, with no component noticeably stronger or weaker than its mates. If the system is unbalanced in favor of the amp, you risk damaging your speakers. If your speakers are liked to a substandard tuner, you'll hear every snap, crackle and pop of poor FM reception and other unwanted electrical noises.

In the smallest of technical nutshells, you require enough amplification (measured in watts RMS -- the measure of continuous power -- per channel) from your receiver to drive a particular pair of speakers, but not so much power that the speakers get blown out. Unless you're really particular, you're probably better off with "high-efficiency" speakers, so called because they make better use of the power coming from the receiver and thus do not require a more powerful -- and more expensive -- receiver, as do their opposite, the "low-efficiency speakers.

When you shop, take along the owner's manaual from whatever component you intend to keep, matching the component's capabilities with the equipment you may buy. If you don't have your owner's manual, at least know your brand name and model number. Further, request a no-obligation trial in your home to insure that the new will enhance the old. PACKAGE SYSTEMS TO GO

If you can afford the luxury of buying your second system as a system, the problem of compatibility won't be so tricky. Most audio dealers put together package systems that are already well matched and attractively priced, so your attention can turn to cost, quality and whatever fine-tuning you want. This might mean learning the statistics of such desirables as low distortion, high signal-to-noise ratio (which essentially means you get a clearer signal) or wide frequency response (as complete a range as possible of audible sounds).

In any case, prepare yourself before making your first wide-eyed appearance at Attila's Cut-Rate Audio. Get as much sound advice as possible, then cross-reference the data to determine your best bets. For starters, you might spend twenty minutes at the local library, jotting down product recommendations from consumer magazines and stereo equipment review. Also, watch for dealer ads in your Sunday papers for a month, noting price ranges for equipment you might be interested in. Get advice and recommendations from knowledgeable friends. Seek a long-term warranty from the store. Most warranties today are for one year, but some equipment is guaranteed for only 90 days. On the other hand, strong service programs provide up to five-year in-house warranties on their products. That's insurance worth considering -- but only if it's in writing. QUAD IS DEAD

Nietzsche didn't say that, but it looks as if the four-channel "quadraphonic" listening experience will not, after all, supplant stereo. Some salespeople still may try selling you more than you need, so beware of the pitch for quad: It's virtually passe and certainly not practical or necessary to serious music enjoyment.

It's difficult to predict what might happen in audio research in the next few years, but if you intend to "trade up" again someday, it might help to keep abreast of certain trends in the industry. The quad demise is one. Another is that equipment of the future may well be modest in size but mighty in performance. So don't let the eager dealer oversell you on size.

Digital and computerized components will surely arrive. Programmable turntables and cassette recorders do exist, but they're only for serious stereo buffs. At the push of a remote-control button, tomorrow's music playback machinery may switch between Keith Jarrett and Steely Dan as easily as today's televisions flash from "M*A*S*H" to "Three's Company."

Lightweight, "on-the-ear" headphones have been used on space missions. They're inexpensive ($129 and up), and are setting new trends for isolated, comfortable listening. If recording is in your future, take note that cassettes are coming, eight-track is going, and reel-to-reel will always be here. TO TRADE OR NOT TO TRADE

Some vintage, top-of-the-line equipment from five or ten years ago still retains much of its value. But chances are, if you're trading up now, you didn't have enough bread for state-of-the-art Klipschorn or KLH back in 1970. And if your first system was less than immortal, you can expect a skimpier trade-in credit. You're largely at the dealer's mercy, but consider this: Even if you're standing at the cash register about to complete your trade-up deal, you don't have to sell the old system. If it's seriously flawed, unload the goods while you can. But if your old gear is still in good working condition, and the dealer's trade-in allowance is just too painfully low, why not keep it? The speakers could serve as extensions on the patio or in the bedroom, or the whole ensemble could be used at that beach house or ski condominium you're hoping to buy someday. If nothing else, you could give the rig to someone you like. Your old equipment could bring out the artistic depths of Mom's Julius LaRosa records for years to come . . . or serve as a starter system for some young Kiss fan in the family.

On second thought, take the money.