In what appears to be an ongoing relationship, Morales was the guest of the Kalichstein-Laredo-Robinson Trio (call them K-L-R for short) at Tuesday's Philadelphia Chamber Music Society concert. In a program opening with the Beethoven, it was hard to say who could be most thanked for projecting its slender charms - cellist Sharon Robinson with her febrile assertiveness, Joseph Kalichstein with ultra-crystalline piano sonority, or Morales with his creamy tone.

Together, they found a surprising amount of richness in the final movement's theme and variations (a form that was Beethoven's safety net). That movement also played wonderfully off of Tchaikovsky's strenuous and more emotionally discontented workout in that same form in his Piano Trio Op. 50, which cleverly framed the program in the second half.

The Kimmel Center's Perelman Theater was packed with an audience that seemed particularly keen to see Kalichstein's keyboard from the first tier's aerial view. His performances never pass without his own heatedly observed, coolly projected phrasing, plus balances with the other instruments that leave you wondering how he did it.

But it must be said that the rest of the trio was in particularly good form; violinist Jaime Laredo seemed on top of technical matters in ways I haven't heard in recent years. And, as Tchaikovsky fans know, all participants must assume heroic stances in the Piano Trio, which sometimes seems like a concerto for all three soloists. The piece has everything - haunting, ethnic-tinged melodies, an exciting fugue, and a balletic variety of incident - but conciseness. The performance intelligently sustained the long musical spans, often thrillingly.

Hornist Radovan Vlatkovic was announced to play the Brahms Trio for Piano, Violin, and Horn, but canceled. The void was filled by Mendelssohn's Piano Trio in C minor Op. 66, to curious effect. The beauty of K-L-R is three large personalities converging on a single piece with unified purpose. Temperamentally, these musicians aren't miniaturists. And that's what Mendelssohn seems to call for in this strictly contained piece, in which "allegro" is code for going nowhere passionately.

Kalichstein had some arrestingly noble moments in the opening of the "andante" movement, but much of the rest of time, I had an image of these three trapped in a meticulously decorated Biedermeier doll's house, bumping their heads on the ceiling, unable to get comfortable in the tiny chairs. This is a compliment. Pushing against boundaries is far more engaging than shrinking from them.