I've used a MisterX CK2III for most of this year with HD600's, driven by either an 0404 USB or a Marantz SA8001. Previously, I owned a maxed-out Shellbrook desktop PIMETA and an n_maher Millett.
The CK2III is undoubtedly the most refined of any of the amps I've owned, and matches up very well with the HD600's. As to the bass.....for well-recorded classical/acoustical material, it has a very tight and accurate low end; I don't find it lacking in any way. Badly mastered pop material might need a little EQ at times, but the overall accuracy on music that is recorded properly, to me, far outweighs its slightly light character on pop/rock.
I like the size factor of mine....2"H x 9"W x 7"D, all in one box. No external PSU box and extra cord to get in the way.
And yes, it does get a bit warm at times!