Might check the cap value on my D-Moll. You say it was on gain pot, not volume pot? I tried to do a search on bright cap mod but of course no Diezel posts. One site talked about cap on volume pot.
Edit: Just checked D-Moll. Nothing on pot tabs except leads going to board.
Yup, on the gain pot so that essentially you're allowing an amount of high frequencies to bleed across the gain pot, the cut-off frequency determined by the capacitor value (the higher the value, the more low cut). By reducing the relative amount of low end hitting the saturation stages, you get a progressively tighter, less fuzzly sound.
When the pot is turned really low, you're mainly hearing the treble frequencies that are bypassing the pot, but as you turn the pot higher you're allowing more of the full range signal to blend in until when the pot it fully open the treble bleed is basically not doing anything. For this reason you have to balance the capacitor value against where you normally set the pot - if you typically use it in its lower range you should use a conservative cap value so you don't end up with a very thin sound; if you have the pot set wide open most of the time, you'll need a high value capacitor so its effect can still be heard.
Putting a capacitor across the channel volume pot will still act as a treble bleed in the same way as above, but since in the case of the Diezel this is after the preamp gain stages you won't be affecting how tight the drive sound is, just creating a situation where your overall tone gets brighter the lower you set the channel volume. The gain pot is essentially just an input volume control, while the channel volume is the preamp output volume control.