This amp is not under $600 but:
I just had a long listing session (3 hours) with the HE-500s and my Balanced M^3. This combo is really great together. The bass the M^3 provides really works very well with the HE-500s. The hot treble on these headphone became non existent and there is no sibilant sounds at all. The Midas became even more lush. Overall I'll say this is the best amp I have thus far to pair with the HE-500s.
Thanks fo that tip prep!
But I cant go over 600 for now. The Sa-31 is the very limit Id rather stay single ended for now... Balanced for my unbalanced sources would be overkill for now