Having now had over 200 hours on the Utopias, I feel more confident that what I write here will continue to be my perception for a long time. I have a hard time believing that the phones will change much more than they already have.
Flat out, I prefer the Utopia with the Phonitor 2. If you want a warm sound, this wouldn't be a good combo. The amp seems to be pretty darn neutral, with a lot of detail, wallop and drive. I think that it brings some of the best out of the Utopias without adding much. As with any headphone amp I have paired these phones with, your choice of volume can be critical to experiencing the best. The Utopias are very dynamic sounding and, if you aren't careful, it's easy to have the volume a bit too high.
Everything that I wrote about the McIntosh MHA 100 remains consistent to now. The combo of McIntosh/Utopia does sound a bit clearer than it did when the headphones were at less than 100 hours. However, the impression of being overly warm in certain frequencies continues. I do notice that in my search for audible sounds that I know are there in the recording but just seem pushed back a bit leads me to have the volume too high. When I dial the volume back just a bit, I continue to feel that the colorations of the McIntosh don't work that well with these cans....certainly not for the price of the amp.
BTW, I was pretty surprised by the differences I heard over time as the Utopias were being "broken in". I really thought that I wouldn't hear much change after 50 hours but that turned out to not be true. The change I'm hearing most since about 100 hours would be covered by the words "becoming even more refined". I spoke to a friend who sells the Focal Utopia speaker line (but not the headphones) and he wasn't at all surprised by what I told him. He said that his experience with the speakers is that they continue to change for at least 200 hours.
I think that I have given as much info as I'm likely to have. I did decide to wait to get a chance to listen to some different amps other than the Woo I mentioned in past posts. I know how wonderful the Utopia sounds with the Woo but I'd like to hear some amps like the Pathos Aurium. I may still buy a Woo but I'm becoming more and more impressed with how the Utopias sound with the Phonitor 2......which I already paid for!