the amps i've heard that sound great with the Atticus (all my personal opinion based on my own tastes, yours will vary):
Violectric V100 (which sound identical to the Lake People G109, and RS series, so all those would be options)
Violectric V280 via balanced output only. single ended is a bit too smooth and warm for my tastes
Lake People RS-08 (a V100 on steriods, same sound signature, more dynamic range and driver control, imo THE best amp for the atticus i've heard, and i regret selling it to this day, and i will be buying another)
Arcam Rhead
Gilmore Lite MK2
Auralic Taurus MK2
Bryston BHA-1
Ragnarok
The above group just sounds good. i wont add individual notes as they'll all say the same basic thing: instantly engaging, but not fatiguing. endless dynamic range and driver control. excellent synergy. you cannot go wrong here (for my tastes!)
Good but not great (synergy and personal tastes heavy at play here. for some, these could go in the top group):
Jotunheim via balanced output. it's pretty good overall, very dynamic and lively, but the treble glare i was hearing from my jotunheim was eventually a deal breaker for me. i had a very early jot, and i've read there was a silent revision to the amp which smoothed things out a bit, but i have no firsthand experience on that, sorry.
Violectric V280 via single ended output. this is essentially a V200 if using the SE output, and i found it to be just too smooth and warm for the atticus. it just didnt wow me like the v100 did.
RNHP was good but lacked headroom and high volume grunt. as the volume went up, the sound got a bit ragged and sloppy. i think the amp was hitting it's output limits, and distorting. at low/medium volume very nice sounding though. i'm a loud listener so it didnt work out for me here, but if you're not trying to kill your eardrums, it would work just fine.
Mjolnir 2 various tubes this is my buddies and i've logged hundreds of hours on it. my thoughts on the V280 single ended apply here. it was just a bit too polite compared to other amps i've heard.
Woo WA3 various tubes this one was tough. it's my buddies and all his tubes are of the traditional/syrupy type (per his assessment, he knows tubes way better than i do). this thing was very warm, like the V280 single ended, but more engaging. i almost put it in the top group, but i could only do this pairing for short bursts as it eventually sounded too warm and overwhelming. but at first listen i was quite impressed. this one has me so curious with neutral tubes that i'm on a mission to grab a used one on the cheap
Not good (remember, just my opinions!)
Lyr 3 new production TS tube just too boring overall. it was nice and neutral, and very clean sounding, but lacked dynamic range. i was underwhelmed.
Asgard 2 not bad, but like the Lyr 3, it was ho hum. i felt bored by the pairing. more treble hardness than the lyr 3
Grace m9xx i sound like a broken record, but much ho humming here too. the grace dac is great, but the amp section just wasnt pushing the atticus well enough for me. dynamics were lacking, and i was bored.
Dragonfly Red lacked headroom and dynamic range. not surprising given it's modest power output. it had plenty of volume, but had issues with driver control. bass got boomy, and treble had a slight glare
Fulla Schiit 2 my notes on the DFR apply here, just doesnt have the power, sounded dynamically limited
Magni 3 a return of the ho hums! plenty of power, but plenty boring. it actually sounded much like the lyr 3 to me, but less refined. same tonality and timbre though, with a bit more metallic-ness up top.
i've heard a few others in short bursts, but all the amps above i've logged at least a few hours with, with my atticus.