"The closeness of the audience, the fact that people are in a much more relaxed setting . . . that's what concerts should be like," said Dinnerstein later. "In concert halls, the separation from the audience feels antiquated to me."

Even the in-concert chat felt different. Performers often talk to audiences these days, but here, the combination of wit and smarts suggested that this pair was addressing friends rather than strangers. Commenting on how Beethoven's sonatas have three working parts spread between the two instruments, Dinnerstein said, "I feel like my left hand should take a bow."

Though Dinnerstein and Bailey happily move on to a season of high-profile conventional concerts (she's in Philadelphia twice this season), the Poisson Rouge advantages were tangible: Afterward, roughly a third of the 250 paying customers bought the pair's new Telarc discs of Beethoven sonatas - in contrast to, say, Philadelphia's Mann Center, where a noted violinist can give a beautifully received concerto performance but stand idle at the autograph stand amid quickly departing listeners.

The audience was unusually committed to being there, as opposed to showing up because the concert is on a long-ago-paid-for subscription. At Poisson Rouge, an advance sale of 30 is a lot. That means the audience acts on a be-here-now impulse - and despite the potential distraction of dinner, was quieter than a Carnegie Hall crowd, a dressed-down version of which was there (and inquiring about disability seating).

This venue isn't unique. Though classical music isn't supposed to make you want to drink (quite the opposite), that option is increasingly available in trendy New York settings better known for indie rock.

While Poisson Rouge casts its net widest - from the ultramodernist string quartets of Iannis Xenakis to Monteverdi's opera The Coronation of Poppea - the stylish Galapagos Art Space, a cabaret in Brooklyn's Dumbo neighborhood, hosts new works being developed by the American Opera Project. And the Highline Ballroom in Chelsea featured a recital by pianist Jeremy Denk in a brainy program of Bach and Ives. True, martini shakers intervened during Denk's Bach, and a bouncer ushered him out when he lingered too long after performing while the club was cleared for the next show. But the freedom of the situation, he said, bordered on eerie.

"I could have put the intermission anywhere. I could have improvised between pieces. There was a lot that I could have done but didn't do because it was my first time," said Denk.

Philadelphia is more than flirting with classical music in clubs. Astral Artists artistic director Julian Rodescu is currently investigating club venues for the organization's young-artist roster. New-music groups such as Relache and Network for New Music have sought out alternative venues, the former in the Prince Music Theater's cabaret space, the latter at galleries in Northern Liberties. Counterculture-minded cellist Matt Haimovitz has played Bach at the Tin Angel and Boulez at the World Cafe Live. The need for more may be undercut by what's already here.

Composer Andrea Clearfield has held a monthly semi-public salon in her loft for more than 20 years - for creators trying out new work and performers who just want to make music under circumstances even less formal than a club. Though Poisson Rouge ticket prices ($10-$30) are uncommonly low for New York, Denk's Philadelphia fans might be perfectly satisfied with his Oct. 28 recital at the Kimmel Center's Perelman Theater (presented by the Philadelphia Chamber Music Society) for only $23, with no drink minimum and safely assigned seats.

Still, the power of the club atmosphere is undeniable: Poisson Rouge's expansive amphitheater layout somewhat recalls the now-defunct Rainbow Room, while Galapagos' main floor is actually a reflecting pool with island-like seating areas.

Despite the temptation to give shorter, lighter programs in standard hour-long club sets, Dinnerstein and Bailey opted for a full evening of three Beethoven cello sonatas with a short intermission. "It felt like the audience could handle a lot more," Dinnerstein said. And she was right.

A certain cultural machismo works in favor of the performers: While traditional audiences may shrink from Ives, some contingents in the club crowd want to show they're tough enough to take it - within reason. Though the American Opera Project is contributing the experimental, Holocaust-oriented Darkling to the current Live Arts Festival/Philly Fringe, the piece will never be part of its semiannual Galapagos showcases. "Most people with a drink in their hand would say, 'Oh God, gimme another,' " says executive director Charles Jarden. "We want them to sip slowly, not down their drinks to deaden the pain."

The lack of a standard program format in clubs can solve lots of problems. Few pianists have found suitable program mates for Bach's Goldberg Variations: Also, the piece's specific technical and mental demands leave little brain space for anything else. That's why Denk chose Symphony Space (not a club but an alternative venue on New York's Upper West Side) to play the Goldberg Variations by itself this spring: A fuller program simply wasn't required.

Carnegie Hall and Lincoln Center, however, are hardly threatened. One reason is that the large classical venues often require an exclusivity clause from artists that keeps them from playing within a prescribed geographical radius, although exceptions are often made upon request, says Ronen Givony, who programs classical music for Poisson Rouge. More significantly, the artists don't earn a lot. Some get as much as 65 percent of the door, says Givony, "but even if I gave the Arditti Quartet 200 percent of the door, it would still be the lowest fee of their careers."

That's why the likes of violinist Gil Shaham or soprano Danielle de Niese tie such appearances to high-promotion moments, like the release of a new recording. If nothing else, the novelty of playing a club often attracts more media attention. Since clubs book only months in advance, they can be up to the minute.

Givony got an online look at the score of the recently premiered, highly hypnotic Symphony No. 4 by Estonian mystic Arvo Part and lost no time assembling what would be the New York premiere - beating the piece's commercial Los Angeles Philharmonic recording by four months. Realizing it could be performed by 30 players, he asked his classical contacts, "What would it take to pull this off?"

"We found a conductor, contracted the players, and presented it in May," he said. "It was kind of special."