remember that a lot of true reference headphones are open-backed, and therefore offer no isolation. this may be a problem during tracking.
that said, the 650's are a great choice. i prefer, however, the grado rs-2's through the grado headphone amp. the grace headphone amp, however, is a very close second (analog side... the digital lacks the silk in the top end).
and, yes, if you're going to spend any amount of time under the cans (especially listening critically to a mix), you'll really notice how much of a difference the hp amp makes.
"My job is to make music sound great and to not whine too much." --George Massenburg