It turns out the Denon AH-D9200 headphones are really easy to drive so play nice on most amps. First world problems, but I was hoping to upgrade further to get even better sound out of them. The ADI-2 can EQ which I've been playing with as well, that's going to take a while. Each upgrade I've done in the past is usually a noticeable improvement in some way and so are the new Denon headphones, but I couldn't really put in words how!
There is no shortage of stuff to upgrade to, just depends how much argh
Apparently, transformer coupled amps might be the way to go with the Denon's but it sounds like it would be cheaper to just get a different pair of headphones lol
I'll do some research but might leave things a couple of years and just enjoy this new setup.