Got the Yamaha HPH-MT220 today. Working on a full-length review, though I may give up and never publish it. lol
In my opinion, the 220 has less problems than the FSP and 7506. Far superior build quality and comfort. Unlike the FSP and 7506, I don't feel the need to push the ear pads closer to my ears to enhance the sound. In fact, when I do that, the sound doesn't change at all. The trade-off is that my ears press up against the drivers at all times, which is the only minor comfort issue. Bass sounds less boosted than the FSP, but still substantial -- perhaps still boosted a little above neutral. No major problems with the treble. But the mids are too recessed.
This review says that after burn-in, it becomes much more transparent. Here's hoping. Despite the issue with the mids, the 220 sounds the most neutral to me overall, since the 7506 is too bright and harsh in the treble and the FSP has too much bass and not enough mids and treble. 7506 has better vocal reproduction. FSP still has better physicality. Tons more info I could share, but I'll save it for later.