I've been doing some mods to mine and thought I'd share in case anybody else is still interested in these.
First thing I did is ripped out all the circuitry inside. These consisted of a thermal protector (in the signal path...) and a resistor network. The resistor network basically kept current in check and also attenuated the signal reaching each of the speakers.
The H2 are actually 3-way headphones.
The strip down the middle is the super-tweeter. I have no idea how high these things go but I suspect they go well beyond 100Khz. The narrow wafered section at the top is the tweeter/midrange, which goes from roughly 4khz to beyond 20khz. The woofer takes up most of the room, and is the deeper set of wafers on the lower majority of the headphone unit. The bandwidth of this depends on the series resistance you use with it. The woofer itself is basically a 55nF capacitor.
So, I removed the super-tweeter and removed the cabling for the tweeter/midrange, thus disabling it. I use a 15R (5W) in series with the woofer, and surprisingly I get the full audio bandwidth. It definitely goes up to my limit of hearing without significant attenuation, which is 17Khz. I shudder to think how high that super-tweeter goes.
So now I can hook it up directly to my crappy speaker amp which puts out about 45Vpp and get a pretty decent volume.
I'm also in the process of doing some structural mods to it, since piezo drivers need a rigid support to work at their best, and plastic (especially the kind of low elastic modulus plastic used in the H2) is not it. So far I've added steel supports to each wafer of the woofers:
I'm now in the process of re-inforcing the sides of the unit with steel.
As to the sound, I don't want to say too much before the mods are complete, but I did some testing after installing the rods to make sure everything was ok, and found it hard to stop listening hours later. There are a lot of changes to the sound compared to the original, but I'm trying to suspend judgement until it's all finished.
I think a balanced beta22 putting out about 90Vpp could drive these to a fairly high volume and do so exceedingly well. Any b22 owners want to volunteer their ears?