Guitar + Bass + Fiddle + Mandolin = the Creaking Tree String Quartet.
I went to hear them last night for the first time. Astounding! Each player is a master of his instrument (I can't say I've ever heard better fiddle playing in my lifetime), and they listen to each other with ease, playing in a style with no name. Not what I'd call 'easy' listening - it's pretty deep and intelligent - but full of nuances and rhythm and jazzy and bluegrassy and definitely engaging. Do give them a listen: http://www.creakingtree.com.

~black walnut