Just to clarify, the channel volume is just a straight up boost, it doesn't color the tone at all, so turning down the channel volume shouldn't cause you to lose any balls. If you have any post-amp effects though, turning down the channel volume reduces the signal that those post effects receive, so something might be happening there, but we don't usually place effects in post when they affect the ballsiness of the tone, generally speaking.

If you can hook the HD up to a signal meter or a DAW via USB, check that no effect or amp input or output exceeds -12dB. Can't comment in general on most effects or amps but the 5 amps I checked and Studio and Mid Focus EQ all start to compress, then soft clip and finally distort if you violate that rule of thumb. Channel Volume is only tone neutral if you do not violate this rule for the amps I checked, but granted it's not a whole lot i measured ... may be worth trying though (I checked the BF's and Soldano's).

The Mixer seems to be an exception and can boost levels higher than -12dB w/o impacting the sound (maybe its +12dB upper limit is a hint of where the signal should normally be ;-), but watch out if the Mixer is not last because every effect that follows may cause the fizzy/distorted sound you report if the Mixer pushed the signal above -12dB. Someone mentioned that before. Move the Mixer to the end if you want to max out the output level.

By the way, lowering Channel Volume from 100% to 50% reduces the output level by about 12dB (that's a little more than half the volume in practical terms, 10dB is the common half/double volume guidance that works for most people). Going down to 25% cuts it about another 12dB, and so on ... The exact numbers depend on whether you already are in compression territory (above -12dB) when you start.

If you can't measure levels I'd go for the listening test in step by step fashion: Go from input to output: Set Input 2 to something w/o a signal to cut it by 6dB and stay on teh safe side. Disable all effects and the amp and set Mixer to 0dB gain, channel A Center, channel B muted. Listen to your raw guitar volume. Should be very clean. Turn everything on 1-by-1 and listen after each effect or amp you enable. If volume changes turn gain or Channel volume down or up. Check groups of 2 or 3 at a time later to make sure you are not consitently guessing on the high or low side and redo the individual step adjustments as needed. Once you hit the end you should be around -20 to -10 dB. If Mixer is at the end use it to adjust to what you need to max out volume or stay sane at home ;-).

If everything is to your liking (missing fuzz or unwanted distortion) and you want a little more volume go back to your first effect and slowly raise the gain (you might have had headroom below the -12dB 'limit'. You may have to reduce the Mixer gain to prevent digital clipping from it. I think this should give you the best leveled signal path w/o measuring anything.

Obviously this is not perfect ... and on't forget the headphone thing mentioned above. You don't want to go through the level matching hassle if its just the cans.

Good luck,

Martin