In terms of sales, the open-reel tape deck has taken a beating in the last few years from the proliferating and ever-improving cassette format. The number of open-reel decks has dropped in proportion and the open-reel format has become pretty much the favored one for a constellation of buyers known variously as "semi-pro" or "creative audio buffs" or "advanced hobbyists."

At least two major companies are still wooing the consumer market with updated versions of open-reel tape recorders that, in terms of performance and features, could appeal to the audio specialist as well as to the broader consumer market.

One is Pioneer, whose model RT-707 I reviewed here in December 1977. For an open-reel high-quality deck, it was surprisingly compact. It offered excellent audio performance, and it had the added fillip of automatic reverse on playback, which, among recording pros, is regarded as a "strictly-for-the-consumer" feature. Be that as it may, the reverse playback was handled by a separate (fourth) head so that the three heads (erase, record and play) used for normal left-to-right tape movement were not compromised.

Since then Pioneer has released the RT-909, a beefed-up version of the RT-707. The most obvious difference in the new model is its ability to handle 10 1/2-inch diameter reels (standard NAB size) which hold twice as much tape as do 7-inch reels. Loaded with 3,600 feet of tape and running at 3 3/4 ips, a 10 1/2-inch reel can play stereo continuously for six hours. Great for operas, among other things.

Not to be outdone, Teac has a new open-reel deck -- the model X-10R -- that not only plays in reverse but also will record that way automatically. The no-compromise approach is exemplified by the use of six tape heads -- three heads for the forward direction and three others for reverse. This deck also runs at the two speeds of most interest to home audio buffs, 3 3/4 and 7 1/2 ips, and it too accepts the large 10 1/2-inch diameter reels. And for those who have no need for 10 1/2-inch reel capacity, there's the smaller TEAC X-7R with the same bidirectional recording and playback facility using 7-inch reels. FEEDBACK

Q. Why would the same record played with the same pickup, but on different turntables, sound different? Specifically, on one machine it sounds as if the bass tone control had been turned up to an unbearable level.

A. Assuming that the bass control had not been so adjusted, and that the speakers are not "boom boxes," a likely explanation of the heavy bass is the low-frequency resonance set up by the arm-and-pickup combination. With some setups, this resonance -- while below the normal audible range -- can interact with another subsonic resonance caused by very small amounts of record warp to create a low-frequency "bump" that in turn reinforces an otherwise inaudible rumble from the turntable and generally accentuates whatever "groove rumble" may have been imparted to the record during its original cutting. Once stimulated, such resonances can intrude into the audible response range, causing unnaturally heavy bass and distortion. In extreme cases, the stylus will mistrack the record.