On paper it looks like a pretty comprehensive and ultimate solution. Balanced outputs, stepped attenuators, remote control, DSD. Everything including the kitchen sink. Now the question is, how much will it cost? Are they using Muses02 to cut costs, or because they sound better in this particular configuration compared to the Muses01?
From what I understand (feel free to correct me!) you will always have a point near the bottom of the volume scale on analogue "infinite resolution" knobs like this where there's a bit of imbalance. To alleviate this issue the gain on the amp should be done in such a way that the user won't have any reason to use the range where this would be noticeable. I assume Sennheiser tweaked the gain specifically for the HD 800 as you'd struggle to hear that point even in a dead...
Even calling it a "problem" in my case feels like an exaggeration. More like a fact of life for the majority of these types of volume controls. To demonstrate the extent of volume imbalance I have, here's a couple images. First is where I hear imbalance, second is how much I have to raise the volume for the channels to be completely balanced.
On my HDVD 800 there's a tiny bit of channel imbalance at the lowest levels of volume, but it's only at the very bottom of the scale. If I turn the knob up just a little bit the channels become balanced like they should, and I can only hear the imbalance if I'm using headphones with high sensitivity like the Kef M500. It might be that there's inconsistent quality on the potentiometers and that you happened to get a knob with more imbalance than it should have.
This is what I'm hearing as well when I compare the two directly. Because the M500 sit closer to your ears I guess they naturally have more of an in your face kind of sound, and their style of treble adds to that impression when you go from the P7 to the M500. It's almost a relief going back to the P7 and having more space between you and the sound and getting the softer treble.
Yeah it's clear that Brent's frequency response graphs doesn't use HRTF compensation like Tyll does, so you get that huge spike in the treble. I find it hard to tell what's going on in the higher frequencies because I'm so used to the compensated graphs.
I don't know what the reviewers definition of "top volume" is, but the bass sounds just fine here when I turn it up to what I'd consider very loud. Whatever volume he was testing that at was probably unhealthy for human ears.