To love baroque-era music is to embrace its lack of standardization. In weekend concerts by Tempesta di Mare and the baroque ensemble Philomel, you couldn't guess what instrumentation was next or in what mutation the piece would arrive.

J.S. Bach's trio sonatas, for example: These works survive in versions for organ (it's a misnomer; there's nothing trio-ish about organ), but are thought to have begun their lives as chamber works, ones Tempesta di Mare speculatively reconstructed for two to five players.

"So they're admitting to playing fast and loose with the music?" one listener asked after reading the program notes. Is that a problem? Obsessing over the letter of the score is so 20th century. The transcription for lute and harpsichord might not have occurred to Bach (the two instruments create sound in too-similar ways), but the timbre was alluring, even if it's not interesting enough to hear again.

From this comes another confrontation with modern thinking: The baroque era didn't have repeated hearings as we know them. So the central question in the two concerts is, "Do you feel good now?" Yes.

Philomel has its ups and downs, but Sunday's concert at St. Martin-in-the-Fields in Chestnut Hill was as up as I've ever heard the group. Guest artist Sandra Miller paired with co-artistic director Elissa Berardi most notably in Telemann's Concerto in E minor with twin soloists, Miller on flute, Berardi on recorder. It was the essence of a baroque music experience.

Mastering the baroque flute is far from easy, but Miller's sense of line was firm, full-bodied, and with phrasing that highlighted key notes in florid passagework. Berardi has the less extroverted temperament of an ensemble player, but the more whistly pitch of her recorder - so similar but so different - matched Miller phrase for phrase, punctuated by Heinrich Biber-like violin cadenzas played by Nancy Wilson with a flair you wish she'd display more often. In less exalted circumstances, the concerto might seem far less interesting; here, it was a frame for performance personalities, and was filled with all manner of incident.

Bach trio sonatas, played by Tempesta, lack the tension of simultaneous voices seeking their independence on a short leash but are full of Bach's trademark melodies, spun through more harmonic mutations than imaginable. At least half the transcriptions were so convincing you couldn't imagine them any other way. Though Friday's performance at Swarthmore's Trinity Episcopal Church needed more rehearsal, co-artistic director Gwyn Roberts, a recorder virtuoso, played passagework so sparkling as to make any fleet-fingered organist seem comparatively lugubrious.

Though Tempesta has excavated worthy, obscure repertoire this season, this less-attention- grabbing endeavor has the most potential durability. For other baroque groups, fresh Bach (even cheeky Bach, as in the lute-harpsichord combination) is always in style. And this is it.

Contact music critic David Patrick Stearns at dstearns@phillynews.com. Read his recent work at http://www.go.philly.com/davidpatrickstearns