On Monday, Jan. 8, the Philadelphia Orchestra, under guest conductor Rafael Fruhbeck de Burgos, will bring to the Kennedy Centre a work of supreme beauty that has not been heard here in over 30 years, if ever. It is the Variations and Fugue on a Theme of Mozart, written around the turn of the century by German composer Max Reger.

The Mozart theme on which Reger built his nine variations and fugue is the opening theme of the piano sonata in A Major, K.331. This music was, incidentally, one of President Harry Truman's favourite compositions, the one that, during a television tour of the White House, he played when he sat down at the piano.

The theme is a simple one, instantly attractive, with a tinge of melancholy. Mozart himself was so taken with it that he wrote six variations on it to make up the opening movement of that sonata. Reger quotes the theme completely at the beginning of his larger work, and then proceeds to add nine variations before the concluding fugue. There is a direct parallel here between Reger's methods and those used by Brahms when he wrote his Variations and Fugue on a Theme of Handel.

But if Reger's music is of such towering beauty, why has it taken so long to reach our concert halls? Perhaps Reger himself had a premonition when he wrote, "Five years after my death, I shall be Forgotten; only later will my time come."

If that comment recalls Gustav Mahler's prophetic "My time will come!" then it may be that, following the recent awakening of interest in Mahler and Bruckner in this country, we will have a chance to hear the major works of Reger played by our leading musicians. Among musicians who know his music well, there is the highest admiration: Rudolf Serkin not only champions Reger at his centre in Marlboro, where sonatas for violin and cello and piano have been recorded; he also made a superb recording of the monumental Piano Concerto, with Eugene Ormandy and the Philadelphia Orchestra.

The Schwann catalogue, that lively barometer of changing public tastes, shows the gradual acceptance of Reger's music.

For years, Reger was not even mentioned in Schwann; now it is followed by a list of 21 items which includes several dozen individual works. There is, for example, the Clarinet Quintet, which most clarinetists with the investigative imagination to play it do not hesitate to place alongside the quintets by Mozart and Brahms. Their judgement is absolutely right.

There are the complete string quartets, among whose passages can be found a profundity of thought and richness of expression to rival those of Beethoven. There are, contrary to the general and uniformed public image (where any exist at all), works of light-hearted humour and wit, such as the marvelous orchestral serenade - though this, unfortunately, no longer exist on any available recording.

Who was Max Regerm whom some of the most discriminating musicians prize so highly, who is well known to every good organist, and often totally unknown to other musicians, though singers should regularly program his original and poetic songs?

He was born in Brand, Bavaria, in 1873, nad died only 43 years later of a heart attack in a Leipzig hotel. (Although short his lifetime exceeded Shubert's by 12 years, and Mozart's by 7.) Reger became a brilliant pianist and organist, a virtuoso conductor - especially of Bach and Mozart, in a day when their music well known to many conductors - and a prolific composer whose final repertoire reached to around 150 opus numbers.

One of his gods was Bach, of whom he said, "Bach is for me the beginning of all music, the solid foundation of any true progress ! A sure cure, not only for all those composers and musicians who are sick of 'indigestible Wagner,' but for all those 'contemporaries' who suffer from any sort of spial atrophy." Another of his idols was Mozart, whom he called "the greatest musical wonder the earth has seen," and he reserved a very special admiration for Brahms.

Reger, who had a prickly temperament not unlike that of Beethoven's, easily lost friends and potential admirers, thanks to a devastating wit. You may have seeen the following comment ascribed either to Beethoven or Brahms. But it was Max Reger who once replied to a critic who had written a scathing comment:

"I am sitting in the smallest room of my house. I have your reivew before me. In a moment it will be held behind me." (Beethoven's similar remark was made in even earhtier language!)

In his student days, working under the guidance of the famed Hugo Riemann in Wiesbaden, Reger would go with friends to a restaurant, ask for "a steak that is two hours long," and wash it down with 20 half-litres of beer. But working "like a horse," as his friends said, he turned out reams of great works for organ, for string trios, for chorus, solo voices, and eventually for orchestra.

Reger had not written a symphony when he died - at the age Brahms was when he finally overcame his reluctance to enter the symphonic world. At the time of his death, Reger was at work on his first symphony. Had he completed it, today's orchestra subscribers might well know his name better than they do.

However, there are plenty of reasons in his two great sets of orchestral variations and the late serenade for us to hear his music more often. The late Paul Hindemith, a composer of unfettered genius and a distinguished conductor, used to insist that Reger's "Variations on a Merry Theme by Hiller" was even finer that the Mozart variation and Fugue.

My own strong preference, possibly based on the extreme familiarity of the Mozart theme, is for this later work. Without offering a detailed analysis of the score, I must say that after the first five variations, which explore the charm as well as the faint touc of melancholy in the theme, Reger moves us more and more deeply into the most expressive implications of the music. This he does through extremely chromatic harmonic thought in combination with the most highly developed contrapuntal technique of his time.

When he reaches the two adagio variations, numbered six and eight, he opens up a glimpse of a very special musical paradise. No other music offers precisely this moment, or the emotion it arouses.

Washington will be in debt to the Philadelphia Orchestra and its guest conductor next week, for this music requires both a peerless orchestra and a great conductor if it is to achieve its proper goal. Two recordings have failed: the first because a good conductor was working with an inferior orchestra; the second because the excellent orchestra had a poor conductor.

It would be a shame if, having rehearsed and performed this music several times in recent days , the Philadelphians should not proceed with the logical step of recording it, either with its guest leader, or under Ormandy's direction. Then, at last, the public would have a real opportunity of coming to know well a great, but greatly neglected masterpiece. CAPTION: Picture, German composer Max Reger: "Five years after my death, I shall be forgotten; only later will my time come."