I've had my Pan Am for about 2 months now. I simply just love it. This is my first experience with a tubes coming from a Headroom Micro stack. It's pretty eye opening (ear opening? lol).
I have a DT880 and an HD600 and was looking at the Bottlehead Crack. But with nary a DIY bone in my body I didn't want to get something I could majorly screw up building. Having very little desk space was also something I had to consider. Ultimately it was Shigzeo's great review that made me decide on the Pan Am. I wanted a Bender to call my own.
What really jumped out to me on first listen was the bass (I put in the Russians first and didn't even bother with the stock tubes). On both my headphones with my previous setup, bass was present, it was well done and I couldn't complain having not heard it presented any other way (I wish I could go to meets). But with the Pan Am, the detail, the quality was something I had yet to experience and I found myself saying, 'wow, I have been missing out.' I was enjoying the bass so much that it was quite a while before I could appreciate all the other things this amp does so well e.g. it really did help taming the highs on the DT880 and just did wonders with the HD600's already lovely mids.
I have since gotten the Mullards to go along with the Russians. I can't say which ones I like better yet. But whenever either is being used I find it hard to switch to the other, so go figure. The Mullards are smoooooooth as silk whereas the Russians are a tad aggressive.
Sorry for the long winded post.
tl;dr I love my Pan Am