I've recently downgraded from my rig to gather funds for a Stax O2 and thus I'm forced to used my modded HD580 out of the Auzen Bravura 7.1 with a Audio-Gd Earth HDAM on it. However, there always was background noise when playing music or audio streams, even with the stock OPA. So I finally sat down last night and figured out how to fix it, and it doesn't even need any sort of hardware modification.
What you want to do is basically set the output of your headphone out to "24 bit, 192kHz Studio Quality" on Windows 7. You can even try and compare it with the default option of 44kHz and notice how noisy it is. To use this fix in Foobar2k with WASAPI or ASIO, you want to use the SoX resampler DSP and resample it up to 192kHz. If you're not using any sort of kernel streaming, just use DirectSound and the changes will apply automatically.
I'm not sure if this is actually driver or DAC related, but at least it fixed the noise floor and could work for other cards as well.
