That is a very strong endorsement indeed.
I admit I have been interested in the HE-6 for some time but until now have left it to one side after reading many reviews that recommend it be driven by full sized amps or even mono-blocks that are designed for speakers. As such I imagined that they would be next to useless driven by the HM-901.
I am encouraged that they could be used by this DAP even if its not at its full capacity.
The sound signature that you describe sounds absolutely ideal to me.
Thank you very much for sharing your experience with many quality headphones that I am yet to audition.
You are welcome.
As you already know there are (many) others who won't agree with my assessments. They obviously experience things differently than me. However, I can say this:
Whether I use the HM-901 with Balanced amp card or via line out into Burson Soloist, Audio-gd Master 6 or even my 50W class A speaker amp, I get the same balance between bass, mids and treble. There is of course difference in terms of transparency, neutral versus slightly warm mids etc. and naturally in max volume, but in terms of balance I hear no difference.
I even took the time measuring the difference between the Soloist (1.7W) and Master 6 (7W) and the measurements confirmed what I heard: No difference between the much more powerful amp and the "barely adequate" amp. Measurements were done at 70dB and 101dB and showed no sign of balance tilt between to two levels with the Soloist versus Master 6. The latter will of course play louder than the former, but in the practical world I don't have any limitations with the Soloist. It might have too little gain in an extreme situation where there could be a very low level track and a desire to play that very loud, but I don't recall having that problem yet.
Consider a speaker setup: You have a 50W amp and speakers with a sensitivity of 90dB. Would you have experts telling you over and over that your amp is way too weak? I doubt so. Most likely you will be told that you are fine, because 90dB is quite sensitive for a speaker and 50W is reasonable to start with. Undistorted peaks in this (speaker) setup will be around 107dB. In the HE-6 with Soloist scenario you will have clean peaks around 109dB (and in addition the impedance curve of the HE-6 makes it an extremely easy load).
Why do people feel the HE-6 is bass shy with lower powered amps? Because in stock condition HE-6 has too much treble and the (earlier HiFiMan) pads doesn't seal well. When they turn up the volume, they will experience more bass and think more power / current is the answer. This is not necessary: Put on the "vegan" pads with double sided tape and you will hear the bass is there. Put felt on the inner magnet grill and the excess treble is gone. Remove the outer grill, foam and fibres at the sound opens up. Wow - what a headphone it is now!
Please note that the
maximum output level with HM-901 and Balanced amp feeding the HE-6 is
limited. It will usually be loud enough, but it won't be able to play
really loud. Modernly mastered pop and rythmical music will be plentifully loud (in my opinion), but if the track(s) are at a low level, there may not be enough gain to play as loud (or louder) than you'd prefer to listen.
This being said, it would be helpful to remember that the required power (measured by InnerFidelity) to reach 90dB with HE-6 is slightly less than 20mW (twenty milli-Watts). 90dB is quite loud as an average level, but then we have peaks that calls for a headroom from a few dB's to maybe 20dB or even more in some extreme recordnings and in these situations you will feel that there is not enough volume directly out of the HM-901.
To sum it up: the frequency balance (including bass level) will be just fine out of the HM-901, but on some tracks you will probably feel that you can't play as loud as you'd like.
