You can find the earlier discussion HERE
This was my search after my old amp died. If you follow it, you will see excellent points from @Torq, @TylersEclectic, @I_want_all_the_tacos, @pwjazz, @generic and others. If my budget were tighter, I probably would have gone with the Atom. Solid state, and with adequate power. I’ve found the HD-6XX really wants more power than Sennheiser lets on to sound it’s best. Not surprisingly, this is something that @Torq told me, but I had to find out for myself.
It’s like the great Will Rogers observed.
There is the one who will learn by reading about it.
There are the few who will learn by observing others.
And there are the many that must pee on the electric fence for themselves.