A little update. I got the Mixamp Gen 1 (aka 2013/2014) and tested it over the weekend with my Fidelio X2, DT770 and DT990 Premium (250). I ruled the DT770 out pretty quickly, for a closed headphone I was expecting a LOT more bass and it didn't really seem to excel at any of the games I was putting it through.
The X2's, as good as I expected them to be, strong (loose?) bass on some games but pretty much like the X1, a strong contender for keeping, only niggle is I don't find them really comfortable but it's not a huge issue.
The DT990's I actually really liked, they were super comfortable and light and although nowhere near as much bass as the X2, they had a very tight bass that still made the games feel alive. The treble seemed quite harsh but this also made some things really clear like footsteps.
I have AKG K712 on the way as I got them for a bargain £144, I also have the K7XX which I will likely move on without opening and try to get my money back.
I am not happy with the mixamp as I have noticed a lot of hissing due to having the volume at max whilst double amping with the Magni. Granted I don't need to double amp the X2 but if I get other headphones in future then I'm in that same situation.
I am going to give the Creative X7 a try, the combined amp/DAC solution appeals to me and it is pretty future proof. I now see that chat issue is resolved with a dongle so it will provide all the functionality of the mixamp + Magni and more albeit for about £150 more than I paid for those two items.