Two Boston-based ensembles of note:
Boston Modern Orchestra Project:
http://www.bmop.org/
The Calithumpian Consort (Led by Steve Drury, based at NEC)
And one in New York:
American Symphony Orchestra-http://www.americansymphony.org/ (Led by Leon Botstein-this guy is a genious and this orchestra has some of the most interesting programming I've ever seen).