Meshuggah. If they are not acoustic/mellow enough, then I don't know what to say.
Edit made it unfunny
how about you try something different than the styles guys mentioned here?.. how about some tango?
try astor piazzolla's "tango: zero hour" or tosca tango orchestra's soundtrack for the movie "waking life"
Comus - 'First Utterance'
Experimental/psychedelic Folk band.. Quite a clear opeth influence, as well documented on terrr interewebz'd