The Isabella Stewart Gardner Museum’s gleaming new Calderwood Performance Hall is an airy, sleekly modern space that feels worlds away from the cloistered confines of the Tapestry Room, where concerts used to be held. The hall is lined on four sides with three levels of balconies, each featuring only a single row of seats. There is no stage; the musicians play on a floor made of Alaskan yellow cedar, with two rows of audience seats surrounding them on all sides for a total capacity of almost 300.