Please take my comments with a grain of salt, because I have not compared the Melos to the ZOTL head-to-head. And my only experience with the SHA-1 is with the Grado HP-2 headphones.
What I remember with the Melos is that it had a kind of warm, glowing sound in mild quantity. I wouldn't call it dark, though. But I think it did "fatten up" the sound a little bit, which is why I call it slower than the ZOTL.
I remember being dissatisfied with the lean character of Stax Lambda Pros I used to own, and then I put the SHA-1 in as a preamp. Instantly I discerned the music got warmer, richer, thicker, and slower. Unfortunately, since the ZOTL is not normally a preamp, I couldn't perform the same experiment with it.
The micro-ZOTL seems about as neutral an amp as I can think of. In essence, I think both amps remove the glare that often accompanies solid-state amps. But the ZOTL adds less coloration and slows the music less; it is less obtrusive. It is more faithful to the recording, and I think that that makes it a better companion for the HD600s.
This isn't a perfectly fair comparison since I did not roll tubes on the SHA-1 but am using RCA 6SN7s and Telefunken 12AT7s on the ZOTL.
Honestly, I liked the Melos. But I was not impressed enough with the
HP-2/SHA-1 combo to keep it, and by the time I wanted HD600s again, I decided to try the ZOTL.