check your subways are the entries/exits damaged a little, or loose, also, sometimes the screw holes for those brackets get worn because of force over time and they are not fastened to the PF very well and require a repair, causing the upkicker to perform weak.. I had a similar issue but only with one of mine, it fixed it...just a thought

strange, I re read your original post and you said the audio goes wacky when kicking out? there is a bridge that supply coil power iirc, could be that, just thinking out loud, check f7 fuse holder in the bottom of the cab, is it tarnished, damaged, etc, pull the fuse and push on it, might be causing a resistance issue, if you have other probs when they fire I suppose.. i dont think the filter cap has anything to do with the coil power.. any other weak firing coils?