What amp did you use to drive your HD650? Over at Innerfidelity, they claim the HD650 needs only 0.13mW to reach 90 dB.
The HE-400 need 0.33mW to reach 90 dB. So, mW to mW, you need about 2.5 times the power to drive the HE-400's to an equivalent level as the HD 650. You should reach 100 dB with about 10x the power, and this is very very loud. That is about 1.3 mWs if you are using the Senns, and 3.3 mWs with the HE-400. So you can see that the main reason you need more power, or less, is about efficiency of the phones.
I don't subscribe to the idea that a "bass watt" is more difficult than a "1khz" watt. What is relevant is the resistance of the speakers and the power delivery of the amp. If an amp is "weak", it shouldn't affect bass delivery, only loudness. If the bass delivery did suck, it is more likely that the frequency response of the amp is poor, rolling off bass frequencies that you can actually hear. A lot of people probably think bass "always" needs more power, because of car and home audio. Its not that bass NEEDS more power so much as subwoofers tend to be less efficient than other drivers in a speaker system.
If you are driving a 300 ohm load with the HUD-MX1, that's about 10x the resistance occurring at 32 ohms. You would still have 2-3 mWs however - enough to drive the HD 650s, in theory, over 100 dB. @32-50 Ohms, with 26 mWs available, you might hit 108 dB with the HE-400. A bit less with 17 mWs. But the source can affect loudness to a degree, and so can amp gain and other factors. Plus, if the amp can't put out its max power without distortion, you won't make it far before the sound degrades.
The only way to know if its going to work for you is to try it. If the amp section can deliver its max power without audible distortion, or rolling off frequencies, it would seem that, in theory, it can drive the headphones quite decently. My laptop drives my HE-400s quite well, but certainly not as loud as my amp can. But they sounded about the same since the source is the same.
Well I used the Fiio E9 to power it, but I tried my old traktor audio 2 dac, which could drive it very loud. But it would sound like absolute crap :) It was as if I could hear all the peaks in the music, but everything else was either inaudible(so very little detail) and the bass was very loose sounding.
I don't know why I thought bass needs more power than other octaves. I thought I saw some graph somewhere that implicated that. I must have misunderstood.
I never knew the HE-400 actually needs more power than the HD650, I always thought the HE-400s were quite easy to drive. Apparantly they are only easy to drive relative to other planar magnetics?
Anyway The E9 will provide enough power to drive them. But I still don't understand why there is so much information lost even when the spl is loud enough. This must have something to do with the amount of volt/ampere the amp delivers. It must! Let's ask Tyll.
Thanks for your wise words :)