Any given concert by the Relāche Ensemble is bound to leave some thrilled and others puzzled. The group taps so many streams of musical thought: A program that goes deeply into progressive jazz might leave modernists feeling as if they dropped in from the wrong planet, and vice versa.

That isn't necessarily a negative comment on quality. But happily, this weekend's conclusion to the Relāche season at the Philadelphia Ethical Society transcended any number of aural barriers. And that is a comment on quality.

The title of the program, "City Shimmer With Urban Underbelly," conjures images of those albino alligators said to inhabit the sewers of New York (a myth given "legs" by the Thomas Pynchon novel V). But blessedly, the program was typical Relāche, presenting music that refers to what's commonly heard blaring out of SUVs on South Street but using it in a much more concentrated manner.

The program's world premiere, Lydian Variations by Albany-born, Eastman School of Music-educated Shafer Mahoney, is based on a scale that the composer says is the backbone of jazz - and in any case, encourages all sorts of unexpected musical sidesteps. The piece is written with a let's-try-everything attitude, which means numerous instrumental configurations were exhaustively but not exhaustingly explored with urgency and concision. It was dazzling, and this is from someone who avoids weekends on South Street at all costs.

Tim Grady's Dark Matter and Erling Wold's Close played well off each other as compositional opposites. Dark Matter had a soul-searching viola solo interrupted, bashed and mowed down by any number of violent musical events - all effective - while Close, a dance score, operated within highly atmospheric, homogeneous textures unfolding over layers of rhythmically infectious ostinatos that giddily recall John Adams' The Chairman Dances.

New York downtowner Eve Beglarian was represented by two pieces that left you alternately inspired and discouraged by acts of compositional piggy-backing. The irreverent cleverness of her neomedieval works is inviting, but her Machaut Ą Gogo, a rap-ish version of a chanson by the 14th-century composer Guillaume de Machaut, illuminated nothing in either century it referenced. It seemed like cleverness for its own sake.

However, the William Blake-inspired Marriage of Heaven and Hell was winningly eventful more for Beglarian's contribution than Blake's, cunningly built over dance-club rhythms that, if actually danced to, are so asymmetrical as to leave you with a twisted ankle.