Hey RG, complete empathy for your exasperation
. IIRC you have been running your rig the way you have it patched (minus the gates) for quite a while and never had all these problems ?? My inclination has always been make the patch rockmaster centric not GX700 as in use the GX700 for effects, so either put it in the rockmaster main loop (as it has 4 loop options) (but loose stereo) or use a small mixer and run the GX700 off a mixer Fx send so you can bring it back in stereo into 2 channels, mixer stereo out to power amp. This is a much better setup for gain structure.
For the kick drum to be setting your rig off (or any other really loud noise) implies tubes (and or PUs) being microphonic. There's no such thing as no microphonics in either tubes or PUs, just some less than others. At a certain volume (usually very loud) anything will feed back. Hums are a headache as lots of things can cause them, like having the GX700 PSU close to transformers. But also for some they find the various devices earthing to each other via the rack rails causes issues. I've never experienced this issue thankfully. Another exacerbator of hum (and noise) is old electrolytic caps which is the main thrust of the MP-1/2 noise mods.
Solutions include isolate in the rack (unit and mounting screws insulated from the rails)
New low microphonic tubes (generally short plate tubes are less susceptible)
Recap preamp and amp
Keep input and power leads well apart, if they have to cross, cross at right angle to lessen any induction
Keep everything away from transformers, particularly power amp transformers
Wax pot PUs (which you've done)
Ensure all good quality leads, the shield in leads can also break down over time but mostly for guitar leads as they are subject to physical ware
Keep rack away from vibrations, e.g. not on top of cabs
But you know most (if not all) of this.
When you play really high gain though, it's always on the edge, hence MJMP utilising 2 gates to control it. If I use the really high gain settings in my MP-2 I have to use its built in gate. At such high gain is also where tube microphonics (and hum/noise) are more pronounced.
If hum is from the rockmaster (and I agree that's likely) then it needs a retube and a recap. It's an old unit, the electrolytic caps will have hardened (as they have in MP-1/2 etc.) The 50/50 is also old and will have the same issue. Maybe use your Koch instead? It's much newer and solid state.
Have you considered building a rig around your MP-1?