I just tried my Hifiman HE400i and Sennheiser HD650 on my phone.

Volume wise, it's fine actually.

Audio quality wise it was defiantly muddy, with the highs not being that crisp, and the bass was rather sloppy.

But keep in mind that volume wise, I was able to drive what is supposed to be rather hard to drive headphones on the phone's on board DAC/AMP.

TLDR, go for quality instead of outright power when looking for headphone AMPs.