Interesting, I have a completely opposite experience!
I have the DT880 Pro 250 ohm as well as the HD6xx 300 ohm. I find the HD6xx slightly easier to drive, maybe 10% louder than the DT880. They're a little more mid forward too which helps them cut through.
I also bought a Schiit Magni 3+ headphone amp out of curiosity, and I found it had a pretty minimal impact compared to at least the Axe 3 headphone output. It's maybe a tiny bit smoother sounding.
The FM3 powers both the DT880 and HD6xx plenty well for me. Maybe it's just me, but I don't listen to headphones at a very high volume. Neither of them require more than half power on the output from what I recall.
Anyways, they are both nicely neutral headphones but I find the HD6xx a little warmer/darker which is less fatiguing while the 880 is a little brighter which can be off putting with guitar.