They're getting better. More like

even

better.

That's not faint praise when applied to highest-caliber musicians such as Christian Tetzlaff and Lars Vogt. As an observation on their Kimmel Center recital Thursday, it comes from a high starting point: They previously seemed beyond improvement.

Both have been at the top of the classical music profession for years: Violinist Tetzlaff has recorded the Bach sonatas and partitas twice, while Vogt is a pianist who easily encompasses the big-fisted virtuoso repertoire, as well as the smaller-scale program presented here by the Philadelphia Chamber Music Society.

The centerpiece was Bartok's 1921 Violin Sonata No. 1, a 35-minute monster of a piece that's stuffed with ideas, all used with a density and uncompromising sense of invention that one is likely to hear only in the high noon of a composer's creative life span. The music also contains some of the composer's darkest moments.

From the opening - a cimbalomlike flourish that's folksy but reaches into strange, uncharted territory - Tetzlaff and Vogt followed all of the piece's hairpin mutations, giving the music more shades of expression than I ever hoped to hear. Though Tetzlaff has often been one to explore a piece in minute detail, he seemed to have more sound to work with here, a burnished luster not apparent in previous visits. Has he acquired a new violin? No, he plays the same modern instrument, made by Peter Greiner. His profoundly concentrated treatment of unaccompanied passages in the second movement stand among the great violin moments I've heard.

What set apart the performance as a whole was the absence of struggle. You don't realize how much is there until you don't hear it anymore. Inevitably, past performances have had under-interpreted moments, when the musicians seemed to be saying, "You're going to have to figure out this part yourself." On Thursday, all corners were infused with meaning.

Usually, Brahms ends a violin recital; in this one, Violin Sonata No. 2, Op. 100 began the evening in a relatively quiet performance. It's here that one noticed a difference with Vogt, who has always been able to scale back his sound, but did so while also maintaining a greater richness of tone at the lower end of the dynamic range.

Franck's Violin Sonata encourages performers to give it the hard sell; these two musicians had their own engaging but earnest brand of flamboyance. They were probably better than they seemed at the time; after Bartok, anything would be anticlimactic.

Contact music critic

David Patrick Stearns

at dstearns@phillynews.com.