HEARING a Schubert symphony conducted by Karl Boehm brings to mind Artur Schnabel's famous remark on that composer's piano sonatas, which he called "a safe supply of happiness." For all his authority in the music of Richard Strauss and his commitment to Mozart, it is Schubert whose idiom provokes the truest and most idiomatic response from Boehm. His way with the symphonies takes up about as close to perfection as is possible in our remarkably imperfect world, and Deutsche Grammophon has put us in its debt by reissuing Boehm's Berlin Philharmonic recordings of the eight recognized symphonies, plus some material from Rosamunde, in a "Bargain Box" at exactly half the price of the regular DG label (7270097, five discs).

While the Schubert cycles under Karajan (Angel) and Swallisch (Philips Festivo) are by no means without their own strong points, and the former is quite irrestible in some of the individual symphonies, no one seems to match Boehm in his unostentatious, superbly apt response to the very spirit and heartbeat of every one of these works.

Indeed, one of the few real challenges in any portion of the Schubert cycle has just come from Boehm himself, who has recorded the Fifth Symphony (in B-flat) with the Vienna Philharmonic on DG's full-price label (2531.279; cassette 3301.279). Here the orchestra itself may account for the even more ingratiating lilt in the singing phrases and the peculiarly Schubertian robustness in the vigorous passages. But it is really a matter, as Washingtonians fortunate enough to have heard this team's Schubert concert in October 1979 need hardly be told, of this conductor and this orchestra responding to each other and to Schubert. The Fifth has simply never been better served on records.

The sound is a good deal mellower and more detailed than that of the earlier Berlin version, and the companion work represents an intriguing addition to Boehm's discography. There have been several of these of late -- notably symphonies by Tchaikovsky and Dvorak, with which this conductor had never been identified before -- but the Schumann Fourth (in D minor) is perhaps no less unexpected, and it is even more clearly a winner.

It is a very large-scale view that Boehm takes of Schumann, but not an inflated one. As always in his readings of such music, he manages to let the drama in the score make its own points, without any gratuitous interpretive overlay, and what we hear is exceptionally convincing and durable because it is so exceptionally musical. This coupling may not be a convenient one for collectors who prefer all the Schubert symphonies in one package and all the Schumann in another, but its commanding integrity and persuasiveness easily override such trivial consideration.

At 86, Boehm shows neither the wearines nor the impatience we sometimes encounter in the work of conductors in their ninth decade. On the contrary, he seems to have grown more eloquently and joyously communicative than ever. I hope DG will give us more of his Schumann. And it is time, too, for more Strauss waltzes from the only conductor ever named General Music Director of Austria.

All of DG's new releases now come out simultaneously on discs and cassettes, and the latter medium has made incredible progress in terms of matching disc quality in the last several years. (Recently I heard again Jascha Horenstein's Mahler Third on Advent E1009, whose sound is demonstrably superior to either the domestic Nonsuch or imported Unicorn disc edition.) To many sound aficionados, however, the ultimate in realism is attained only on open-real tape, and those who are not already aware that Barclay-Crocker has begun processing tapes from the Philips catalogue are advised that these may well be the most impressive releases in this format from any source so far.