If you are in NYC and have some spare time, you should make it a point to go to Columbia University's Philosophy Hall and attend their free lunchtime concert series. Right now, Jennifer Koh is going through Bach Partitas, one Partita at a time:
When I was at Columbia a year and a half ago, I attended a few of the Pacifica Quartet's Beethoven Cycle concerts... they played one quartet per concert. I wrote about the C#-minor quartet performance here:
Anyway, it's really casual. Some people even bring kids! Pack a lunch. You can sit literally an arm's length away from the performers if you get there about 20 mins early.