Stability establishes identity.

The Cleveland Quartet's 18 years together with only two personnel changes -- both in the viola chair -- has fostered a rare chemistry and musical consistency amply documented in concerts and recordings. This has been especially evident in its annual venturing of the complete Beethoven string quartet cycle, the 18th of which began last weekend at the University of Maryland with James Dunham, who joined the group in May, as the violist.

The quartet's distinctive sound, a silken heftiness, comes from the Stradivariuses on loan from the Corcoran Gallery. If one believes in artistic transference by acquisition, then Niccolo` Paganini, former owner of these four beautiful instruments, must share credit for the group's virtuosic ensemble playing, notably in two new CD releases: Robert Schumann's Piano Quartet, Op. 47, and Piano Quintet, Op. 44, both in E-flat (RCA 6498-2-RC), and Felix Mendelssohn's Octet in E-flat, Op. 20, coupled with his String Quartet No. 2 in A Minor, Op. 13 (Telarc CD-80l42).

Pianist Emanuel Ax is a sympathetic partner in the Schumann pieces, evidence that all concerned had sufficient time to explore both works fully before preserving their insights on disc. In fact, Ax and the Cleveland took these pieces on a 10-city concert tour (including a Terrace Theater date) last year before recording them at the Eastman School of Music, where the quartet members teach. These readings take a macrocosmic view of the scores instead of relentlessly dramatizing each movement. The phrasing is elegant, the music's direction alertly mapped out so that the melodies flow naturally.

Compared with an earlier version of the Schumann Piano Quartet and Quintet by the Beaux Arts Trio (with Dolf Bettelheim on second violin and Samuel Rhodes of the Juilliard String Quartet on viola, Philips 9500 065), the Ax-Cleveland collaboration seems dainty and a bit contrived. The Beaux Arts favors slower tempos in the inner movements, milking ritardandos and erupting with agitation when the proper situation arises.

The scherzo in Schumann's Piano Quartet, reminiscent of Mendelssohn's famous Octet scherzo, is a sprite's frolic in the Ax-Cleveland account. For the Beaux Arts, this Schumann movement is a Walpurgisnacht crashed by goblins. The Beaux Arts remains excitable, headstrong; the Cleveland serene, imperturbable.

RCA does a mediocre job capturing the Cleveland at full throttle. Heard live, Paul Katz's robust cello tone and the mighty viola of Atar Arad add body to harmonies and a needed jolt to the music when counter melodies or themes pass into their hands. This CD minimizes the impact of Katz and Arad, a shame and a mistake not repeated by Telarc.

A new location and the absence of any signal-processing devices in Telarc's digital production makes a significant difference. The Cleveland recorded the Mendelssohn Octet and Second String Quartet in Houghton Chapel at Wellesley College last November, three weeks after completing the Schumann project. The chapel's lightly reverberant acoustics allow the matched Strads a chance to breathe and breathe fire. Telarc presents the Cleveland very close to the way they sound at the Corcoran Gallery's Hammer Auditorium, their nominal Washington home.

Violinists Donald Weilerstein and Peter Salaff dart and scurry, Katz and Arad play cat and mouse in the opening allegro of Mendelssohn's Second, a remarkably accomplished teen-aged creation. The work has an emotional maturity that belies the composer's youth and offers enterprising musicians a chance to exploit their lyrical side. The Cleveland Quartet seizes every moment.

It follows suit in the octet assisted by the Meliora Quartet. Formed in 1982, this group is a newcomer by Cleveland standards. Yet the pairing of these quartets makes sense, since the Meliora members have studied with the Cleveland Quartet. More important, this combination works musically. The high caliber of play springs not from a dutiful student-teacher tandem, but from compatible equals whose experience and energy rejuvenates a popular, familiar piece. Mendelssohn "invented" a vital chamber format that gave free reign to his imagination. The Cleveland and Meliora Quartet perform the Octet in the same jubilant spirit it was written.