While not specifically with the Bithead I have had noise issues on the USB output if I had my laptop plugged in. This has varied from computer to computer, but might be something else to check out. The bithead I had on loan for a short while (for a Boston meet) was darn near silent in my work rig.
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SB3/Arcam CD73 -> Hertsens Signature DAC -> Project Menace -> Woody HF-1 or HF-2
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