USB powered DAC -> Battery powered headphone amp -> headphones - ground issue

[michalrz]

Hello audiophiles,

I'm preparing a gift for a friend. He recently got himself a pair of some nice sennheiser headphones.
I would like to give him my USB DAC. It draws power from USB. As it does not do any amplification beyond the PCM2704 DAC, I am planning on connecting Left,Right and Ground from its 3,5" jack output to a CMOy headphone amplifier I've once built. It needs symmetrical power and as such it has a virtual ground formed by two resistors.
The CMOy I used to power using a 12V lead-acid UPS battery or a 12V lead-acid cash register backup battery.
And here's the problem: I am quite certain I'd run into issues by having two grounds: one from USB on DAC stage and one from the battery in the amplification stage. Will there be DC offset issues? The connection is unbalanced after all. I'm afraid I'll damage his headphones or something in between.
Could anyone help me with this particular configuration?
Maybe I should use his PSUs MOLEX to power the CMOY amp instead of using a gel battery and thus 'equalize' the potentials between those two devices?