Who would have guessed that an ultra-muggy evening would have the makings of a pleasant outdoor concert? Yet the National Symphony Orchestra's playing Thursday at Wolf Trap, despite the far-from-ideal atmospheric and acoustical conditions at the Filene Center, was refined, unclotted and thoroughly indoorsy.

Lovable mezzo Frederica von Stade, with sweet, teardrop-shaped tones, sang four Mahler songs in the program's first half. At concert's end she and baritone Frank Hernandez sang Richard Danielpour's "Elegies," a five-movement song cycle set to poems by Kim Vaeth, commissioned in 1997 for von Stade. The words were drawn from letters sent by von Stade's father--who was killed in action in WWII before she was born--to her mother. A deeply moving experience, to be sure, though one wished that Danielpour's settings carried more emotional weight and, for all their flow, fit the voices more comfortably.

The improvement of the NSO's string sections month by month under conductor Leonard Slatkin is a continuing source of wonder, and was apparent even here. A tightly knit, homogenized, tonally cozy blanket wrapped Wagner's "Meistersinger" Prelude and Mozart's "Eine Kleine Nachtmusik" and made a warm bed for Faure's "Elegie" (with a patrician David Hardy as cello soloist).

But there's a trade-off with the sort of refinements Slatkin has been instilling with such efficiency: The orchestra's playing, its overall sound, is more beautiful and polished than ever, but it has come at the expense of personality. There was an unnerving blandness in the NSO's sound Thursday, and it felt like an identity was held in check. Some orchestras manage to combine a sweeping, amalgamated sound with a vivid sense of up-close, chamber music-making--the listener is invited to survey a big picture that's as compelling as the nuanced details, with multiple tiers in between.

That's not what happened here, though hopefully playing together is the first step toward playing with unified personality.