This is what you'd have to expect from any gain channel, really. A "gain" channel works by chaining gain stages so the last one(s) are pushed out of their linearity zone - IOW they have to amplify more than they can, resulting in the input signal being clipped. This generate harmonic distortion AND some compression. Your clean channel has some headroom so pushing it with a booster will yield a sensible increase in output volume and eventually some clipping (from 'about to breakup' to mild overdrive, depending on the amp and booster and settings). The gain channel's last stages being already clipping, they don't have enough headroom left (once you're at 11, you can't get no further, can you ?-)) so trying to push them harder will only very marginally increase the output volume, and mostly make your channel clip and distort (and compress) even more.
This also explains why your delay is " massively amplified" (more gain, and the compression effect you get from the clipping) and "with much dirtier overtones" (obviously since your delay signal is clipped to). FWIW that's also why it's often recommended to put time-based effects (delay, reverb, but also chorus and flange) in the FX loop (that is, between the preamp and poweramp) when you use a lot of preamp gain. I don't give a damn as far as I'm concerned - I _do_ like the horrible mess of distorded delay :mrgreen: - but YMMW.
If you want to stack gain stages (ie use a boost or OD before an amp's "gain channel"), you'll get better results by starting with all gains (pedal and amp) set fairly low, then trying different gain settings from both until you find the right "balance" for you.
