I would have thought the Earmax Pro would be able to drive HD650s pretty comfortably. Even the plain Earmax would be okay most of the time - HD6650 impedance is comfortably higher than the Earmax minimum.
As you can see from my sig, I own HD650s and a Graham Slee "Solo" amp, and I think it's a great combination, though I must confess that I've never actually auditioned an Earmax. The 3 "high-end" models that tend to get bandied around are those two plus the Sugden Headmaster, which is very good, but rather pricey. (Good value if you happen to need a pre-amp at the same time, as it does both).
As another possibility, if you're thinking about a CD player upgrade, you could maybe buy a Benchmark DAC1, which will sound as good as almost any CD player in the world, and has quite respectable headphone capabilities as well. (Bargain at the price).
Sennheiser themselves use a Graham Slee amp at their stands at audio exhibitions, incidentally, so
they obviously think it sounds good.