Chamber Orchestra of Philadelphia's "Improvisionaries" season — devoted to the many manifestations of improvisation — stands to illuminate what's happening all around us in the classical music world, maybe without our full awareness. At Philadelphia Orchestra concerts, for example, soloists such as Joshua Bell extemporize in their cadenzas, while any given Four Seasons has a fair amount of harpsichord improvisation.

The Chamber Orchestra went much further in its Sunday concert at the Perelman Theater, with flute soloist Mimi Stillman having a high old time in Mozart's Flute Concerto No. 1 and music director Dirk Brosse guiding musicians and listeners through John Cage's esoteric star-constellation-based directions in Atlas Eclipticalis.

Some of the music was more interesting to talk about than to hear. The musical modules that Mozart wrote for his Musikalisches Wurfelspiel — to be ordered according to a roll of the dice — aren't his most engaging work. And though Cage's Atlas is one of that composer's better-known works, I haven't yet heard a performance that didn't sound wispy and directionless. But here's the twist: Though the piece's directions (based on a map of the stars) leave more up to chance than a drive through Cherry Hill, the recordings I've heard — as well as Brosse's realization of it on Sunday — are surprisingly similar. With open-ended directives, musicians somehow arrive in a similar place with different instruments making short, almost pointillistic statements that correspond with certain stars.

As is often the case even in performances that were supervised by Cage, once you understand how the concept translates into sound there isn't a larger narrative to keep you listening. Stars are portrayed with a refreshing lack of sentimentality, but also without a larger poetic meaning. The piece is best considered "augenmusik" — most fascinating when studied on the page. Worth hearing? Yes — but not often.

Brosse's own 2016 composition DNA in Music took its cues from DNA's simple "nucleotides" to create four-note melodies. But you didn't need to know that to enjoy the engaging suites of movements with ear-catching orchestrations. Process pieces have to pull their weight as a pure listening experience, and this one did.

But as much I applaud orchestras for breaking out of standard concert formats, the more traditional stuff was what most delivered the goods Sunday. The Mozart Flute Concerto No. 1 isn't a great piece, but Stillman's wit, invention, and brinkmanship during the cadenzas delivered one of the better performances I've heard. With her typical steel-spine tone and sense of interpretive purpose, she quoted the composer's Clarinet Concerto and Magic Flute opera, launched into chromatic gestures not characteristic of the period (fine with me), and seemed to keep everybody guessing (including Brosse) when she would end.

The treat for me was Roussel's 1927 Concert for Small Orchestra, with its busy rhythms, small-and-large ensemble interplay that echoes music of Handel's time, plus astringent harmonies and a spirited manner suggesting Honegger on meds. The deeply felt second movement is the heart of the piece and was played at a high standard that has sometimes been missing from Chamber Orchestra of Philadelphia concerts.