well, my system is still evolving cables, preamp and power supplies, and i cant say i have heard the ATM's, but i am sure they are very high quality, but let me say this - my Berning has a very tight and controlled bass, plenty of get up and go - I actually prefer its bass over that in several more muscular solid state amps i used to own. Don't confuse quality with quantity.
My Berning has coaxed a wonderful, full bass response from from my Sonus Faber Guarneri Homage's which dont do that much in the bass department, so it is saying something. (In fact, i thought about selling my REL sub, tat's how good the ZH handled my bass needs.). Also, dont let the power rating dissuade you - those 70 watts go an awful long way. Im no bass-head otherwise why would i own monitors, but I do demand solid control across the whoe musical spectrum.
I stopped messing around with solidstate after persistent sterile "audiophile" sound and gravitated towards tubes to inject real MUSIC into my system - I suspect most tube lovers have had similar experiences. So before the Berning I audtioned tubed amps from Audio Research ,Bel Canto's SET80, VTL's, Cary and then fell hard for the OTL's (atma sphere in particular), but there were some serious practical consideratins - like the 5+ degree increase in my listening room temp, all those tubes to content with, the 90+ lbs of weight per bock, waiting for one of my kids to pour apple juice on the exposed tubes, and so on.... and yet i was about to lay down that cash. Then on a whim, i justbought the Bering (also an OTL, but with some crazy-ass design I cant even begin to understand that surpasses all other OTL's - it's light @ 10 pounds!, reliable and runs like a 300w light bulb at full throttle), never having heard it, and let me tell you, it is one piece that will NOT leave my system. AT 5500, you can not do any better!
Ok, i just realized I am hijacking this thread to wax lyrically about the Berning, and that's not your question -I recall you wanted a speciic comparison that i can't directly provide. sorry for the digression, i just feel that positively about this amp. My $0.02
