It's a shame that there is no USB DAC option directly on the HM-901.
I think a Mini/Micro Dock for travel would be a nice option that would bring it into equal feature set with the HM-801. Really liked the swiss army knife aspect of having so many connection options.
As it stands, you can't do more than two things without the large desktop dock with the proprietary connector on the HM-901. Want to recharge and do line out at the same time on the go? Sorry, can't do that. Want to recharge and SPDIF input at the same time? Sorry, can't do that either. Want to recharge, SPDIF, and Line out to an external amp? Nope. Pick one.
Also, the HM-901 cuts off a significant portion of the first part of every track played, which is made even worse with gapless, Hopefully it gets fixed in the next firmware update.
And the UI while much nicer and more intuitive than the HM-801, is much slower than the HM-801. There is a 2-5 second delay when trying to play another song when navigating to another folder. Which is extremely irritating.
That said, I really like the HM-901. It is a definite improvement in Sound Quality over the HM-801. But it's a big step backwards in features and UI performance.
But in the end, Sound Quality reins supreme.