Indeed, it's an OG pair of 600's that I know quite well as I've had them since dirt was invented haha. Right now, yes I'm using HDV 820 for DAC and amp duties (not to mention the ability to drive multiple sets of cans at once is awesome). The gain is quite predictable with a linear slope in the usable volume range, and this makes it awesome for A/B (and C haha) comparisons. I also spent a lot of time going from IE 900 to the HD 660S2, even though they are fundamentally strangers, all through HDV 820. Although my audio source changes constantly today I was streaming from the S22+ on Apple music, lossless, and some locally stored .wav and .flac from Quobuz. We will be announcing a limited HDV 820 tour shortly, as it's been harder to demo the last few years in the pandemic.