There are mod instructions which state that adding another 9v battery will give you a gain of 6-8 and covers 121-600 ohms. I have personally done the additional battery mod and have Sennheiser HD650 (+600 ohm impedance). I also run a MUSES01 as the opamp. I was warned by JDSLabs that it would work good for only a short period of time while the twin 9v batteries had a good charge. So far I haven't run into any degraded performance but then again I probably only listen an hour or so at a pop.
To compensate I ordered a higher voltage charger (specified in their online documents) to charge the dual 9v batteries in place.
If it gets to be a pain it's easy enough to go back to the original opamp and one battery. I'm just playing around is all.
And I already blew up a U2 rail splitter transistor playing around. No biggie already swapped it. This is my Frankenstein CMOYBB. Pushing the limits of common sense and practicality with this one.
I'll say this for the dual battery MUSES01.........
IT SOUNDS GOOD. DAMN GOOD.