I own both the RKV (Mark II) and the ZOTL. BOth are great amps. I have been meaning to do a in depth side by side of the two amps for a while but never seem to get to it. I actually have both set up next to each other, but with some differences in signal path so absolute comparisons are a bit fuzzy (I'm a real A-B kind of guy), but I'll offer some impressions.
The RKV is gorgeous to look at and has that wonderful liquid "tube" sound in spades-luscious mids and well reproduced highs and transients as well. Bass is well balanced and strong.
The ZOTL looks like a mutant toaster, but is attractive in that sort of utiltarian kind of way. As someone here has mentioned in theri review, the ZOTL sounds much less "tubey", resembling in someways a very good SS amp but with just a touch of that liquid sheen. I completely agree with that assessment.
SOme comments on my setup, which undoubtedly confers some of the differences in my opionions of how these amps sound. My rig for headphone listening is a Cal Delta transport, with balanced output connected to a Birdland Odeon-lite DAC (upsampling DAC, heavily modded by its previous owner, and wonderful). I only use Senn 600's (with new Cardas replacement cables) with the RKV, it doesn't like low impedence phones (Grado RS-2, in my case).
The Delta transport is attached to the ZOTL through a CAL Sigma II DAC (very good but not quite the equal of the Odeon). I generally use the RS-2's with the ZOTL. ALl connections are either Zu cable (the balnced digital cable) or Homegrown SIlver lace (everything else). As curently set up, I listen to well recorded, more aggressive music with the ZOTL/Grado combination (it is very detailed, bad recordings really stink to high heaven with it), everything else with the RKV/Senn's.
Wordy relpy, I hope it helps.