Thanks. This doesn't seem to affect what I'm experiencing, however.
When playing music, instruments sound more natural and real when Windows volume is set very high compared to when it's set very low (e.g., bass guitar sounds very thin at low volume but full and thick at high volume; snare drums sound extra sharp and punchy at low volume but well rounded at high volume). Oddly enough, I'm not experiencing this difference when using a USB dongle: things sound virtually the same whether at low Windows volume or high Windows volume. I've heard that with other DACs, you want to set the source audio volume to max to get the best quality audio, but with the G6 that means it's the volume that G6's amp will output.
Hmm... Now I'm wondering if I should get a separate AMP or try to find a separate volume reducer.