Five years ago when 27-year-old National Symphony Orchestra violist Miles Hoffman sold the Library of Congress on letting him balance out its august winter entries with a little summer season, it sounded like a lark. Perhaps, it seemed, a venture moored more in youthful enthusiasm than in the hope of sustained commitment.

But Hoffman has fooled us. With the passage of time, summer with Miles Hoffman and friends has become an enduring institution.

Tuesday night's opening concert of this season's five, with six players deployed in various combinations, was a suitable index of where the group has gone. It was not just a diverting evening; it was distinguished music-making.

The central work -- Brahms' expansive and passionate Sextet, Op. 18, for two violins, two violas and two cellos -- invited some rigorous comparisons. It was the third performance of the sextet this listener has encountered here in less than two months, and was much the most absorbing. Brahms' lengthy early work is consumed with ardent feeling; it is awfully demanding. The dominant lyric phrases in the first viola, for instance, call for a flood of tone, and Hoffman produced that with splendor (wouldn't you know a violist would open the season with a piece in which that unjustly slighted instrument is so important?). And this same kind of singing sound was suggested in matching phrases of violinist Junko Ohtsu and cellist Evelyn Elsing.

The emotional counterfoil of the Sextet's passion, and of the same trait in much of Brahms, comes in passages of probity and repose. An especially beautiful example: the pizzicato resolution at the first movement's end of the viola's solo theme, like a sort of final deep breath that places all that had come before in perspective. It was played with exactly the right detached breadth.

*Diversity has been one of the special appeals of Hoffman's programming. He came up with a real rarity Tuesday night in a charming little duo for violin and viola written in 1983 by Maxwell Raimi, a young violist -- no less -- in the Chicago Symphony. It has three movements, with little expressive pretension -- and therein lies much of its appeal.

The material of the duo suggests the French composer Poulenc's urbane melding of elegant form with rough-edged street themes, but transplanted from the boulevards of Paris to the dives of Chicago's South Side. Raimi's notion of a scherzo, for example, is a sultry bop sax kind of melody suspended in the violin above a pizzicato bass line in the viola. Maybe the idea sounds a little dumb in print, but not when accomplished with such musical style. Raimi and violinist Bayla Keyes played the piece with conviction.

There was also a fine, and little known, B-minor piano quartet that the 16-year-old Mendelssohn dashed off one day. Its third movment especially -- with much of that inimitable melodic sleight of hand that would soon crop up in the "Midsummer Night's Dream" music -- was masterful. Edmund Batterssby was the splendid pianist.

Three rather severe Purcell fantasias opened the evening.