particularly conducive enviroment for embrasssing the composer's message

There is no place like Merkin Hall for intimacy and sound as well as variety of programing

As a performer who has experience as performer and audience member, my voting, IMHO, would be in order of preference for chamber music programming: Juilliard's Concert Hall; Frick, though small, excellent; Kaufman Concert Hall at the 92nd St. Y; Gilder Lehman Hall at the Morgan Library with 236 to 295 persons , capacity; Grace Rainey Rogers Auditorium at the Metropolitan Museum of Art; and the Manhattan School of Music's Recital Hall, which I remember performing in when the building was Juilliard; Alice Tully Hall; Weill Concert Hall at Carnegie and Town Hall line up in caboose order, only relative in degree to the previously mentioned, but still solidly representative of the performance given.

I know this is extending the question beyond the boundaries given, but I wanted to mention the summer chamber music festival, Music Mountain in Connecticut. The simple, summer-camp-like hall has great sound and, if you sit fairly close to the stage, a great view of the musicians.

One of the definitions of a good chamber music venue is that it should make absolutely no difference where you sit. It's size must be limited by being able to provide an intimate listening experience to everyone.
The first ten rows of Avery Fisher are very good, after that it's dry as dust.
Merkin is terrific when you sit in the audience, but when you're on stage it's very difficult to hear the other people you're performing with. You have to watch them to stay in sync. Peculiar.

House of the Redeemer (Fabbri Chamber Series), 7 West 95th Street, a NYC Landmark.The music room is a wood-paneled Italian Renaissance library moved from Italy almost 100 years ago by Cornelius Vanderbilt's granddaughter. Seats about 120, give or take.

What about Lefrak Hall at Queens College.

Great acoustics and seats 450 people.

Can't believe you left out Zankel Hall - great acoustics (alright a little subway noise), a terrific design and ambiance. It's got Weill beat by a mile.

Two iconic chamber music venues didn't seem to make the list: the Grace Rainey Rogers Auditorium at the Metropolitan Museum and the Kaufman Concert Hall at the 92nd Street Y. While the new Alice Tully Hall is gorgeous, either of those other venues has a history that's difficult to ignore.

The finest venue is the one used by the Jupiter Symphony Chamber Players. It is the Good Shepherd Church on 66th Street, Between Broadway and Amsterdam Avenue. There are no finer programs, musicians or acoustics.

What about the 92nd St. Y? Wonderful chamber music venue.
Also excellent is the concert hall at the Metropolitan Museum of Art.

You left out TOWN HALL - an historic gem with (admittedly) mediocre sight-lines, but fine sound. I find Caspary cold and brittle by comparison. You can have Tully; Weill's good; the Frick divine.
How about the Morgan Library's auditorium?
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You left out the marvelous domed Caspary Auditorium on the campus of Rockefeller University, where free concerts are given every friday at noon during the academic year.