ctaxxxx
1000+ Head-Fier
In my experience with the Atticus, amps with higher output impedance tend to bring out the mids a little more, and amps with low output impedance tend to make Atticus slam a bit harder. HOWEVER, the individual flavoring of the amps also comes into play. Some OTLs slam pretty dang hard. Some low impedance solid states are light on impact. Atticus won't override the inherent sound signature of your amplifier, but it will combine with your amp's signature to author the final sound: an amp that is heavy on impact and low-impedance will stack to make it hit harder, etc.
FYI @ctaxxxx - most mid-priced gear doesn't slam very hard compared to the high-end, summit-fi stuff. If you want to stay in budget range and get more bass out of the Atticus, I'd cancel the Garage 1217 order and buy the iFi iDSD Micro Black Label - it hits really, really hard for a product that can easily be had in the $400-$500 range.
+1 Micro iDSD. I don't own the Atticus, but rather another hard-slamming headphone (Fostex X00), and the original (non-BL) iDSD matches it perfectly. Not only does it tame the X00's highs, it extracts all the possible detail from the bass without bleeding into the mids, and if the song calls for slam, it slams! There's very little difference in terms of SQ between the original iDSD and BL version - I've actually read the BL is a little shier on the bass - so you can't go wrong with either version.
Hmm... I may actually try it. I've always been curious of that amp. It has a hardware bass boost as well. I'll probably buy both and keep whichever performs better. I just didn't want to drop $1K+ on an OTL tube amp that would be compatible for only one of my headphones.