Even though I love my HM-801, the warm mids come with an expense: a slight veil. That's how I heard it when comparing to my Audio-gd Ref 7.1 (and line out from HM-801). And then there is the treble roll off, which is wonderful with HE-6 and other headphones/IEMs with more than neutral treble, but a bad thing with LCD-2 and other dark sounding HPs/IEMs. Also the lack of 88/24 support, the high or moderately high output impedance in all the amp cards - and channel imbalance when listening at lower levels with sensitive IEMs (even with GAME amp).
I should receive my HM-901 within a week or so. In short I expect the HM-901 will provide fixes for all my above issues with the 801. The only drawback with the 901 might - for some - be the less warm mids (as I see it now before actually having my hands on the unit). Hopefully the UI and especially the throwing away of the first second or two in the beginning of tracks will be fixed soon.