Gary Karr did not carry out his instrument when he came back (frequently) to take bows last night with the National Symphony Orchestra.

That sort of thing is well enough when the soloist plays a violin, a flute or even a cello; after all, the instrument does much of the work and deserves a share of the credit. But Karr is a double-bass virtuoso.

Karr seemed happy to leave his instrument (somewhat taller than himself and just barely portable) backstage at the Kennedy Center when it was not being played.

He used it for two numbers: the Concerto in F-sharp Minor by Serge Koussevitzky and the Tarantella by Giovanni Bottesini, in the first performance of a new orchestration by NSO principal double-bassist H. Stevens Brewster. Both works were interesting as novelties, even if they did not plumb any depths of the human spirit or reveal breathtaking new modes of expression.

Fortunately, the program also included a magnificent performance of Mendelssohn's "Scottish" Symphony (No. 3 in A minor)--a specialty of guest conductor Peter Maag, who made it sound like greater music than it really is.

Karr showed that his ungainly instrument can sing with a wonderfully mellow tone and exquisite cantabile line; also that it can produce notes with remarkable rapidity and accuracy when it is in the right hands. His intonation (barring one or two small slips) was precise, and he was alert to the music's considerable lyricism as well as its more modest dramatic and coloristic resources. But the double bass, even with a remarkable performer, remains a rather limited instrument.

The Koussevitzky Concerto, a youthful work, opens with a splendid Tchaikovskian flourish in the brass that makes the rest of the music a bit anticlimactic. Elsewhere, the orchestra is rather neutral, except when the Tchaikovsky motif returns, but it does make a comment now and again to give the solo instrument a rest from its charming, inconsequential melodies. There is a touch of drama in the slow middle section, but not nearly as much as the emphatic opening bars seemed to promise. And there is a dramatic climax at the end, but it seems arbitrarily tacked on to a rather uneventful work.

Bottesini's Tarantella is a display piece, pure and simple. Agility and singing tone are what it is all about, and Karr has those qualities in abundance. Brewster's orchestration was one of the work's major attractions last night--colorful and transparent, giving variety and vitality to the orchestral sound without covering the soloist. Perhaps Brewster should think of reorchestrating the Koussevitzky Concerto as well.

The program was arranged in a roughly climactic order, beginning with a well-phrased but rather subdued-sounding performance of Mozart's 29th Symphony in A and reaching a peak in the chorale-like final bars of the Mendelssohn symphony. For this work (the only one on the program using the full orchestra), Maag discarded the rather bland competence he had shown earlier and became a demon for the small details that have such a strong cumulative effect. He balanced the sound beautifully, shifting the aural spotlight deftly from one section to another as the music required, providing exactly the right degrees of dynamic contrast, modulating the pace exquisitely and polishing the phrasing to a high gloss.

Flexibility was the keynote of this interpretation, in mood and in color. Lilting, solemn, darkly brooding and finally triumphant, the music changed subtly from moment to moment without ever losing its impetus and continuity.

After two weeks as a guest of the NSO, Maag has proved himself an extraordinary conductor when the music interests him and a highly competent one the rest of the time.