Well it's good your getting worked out/isolated
Thump/pop definitely sounds like the power amp, can sometimes just need re-flowing the solder joins around the power supply (though doesn't hurt to re-flow all the joins). I fixed my B200s (loud crackle in one chan) by re-flowing the whole thing (at MJMPs suggestion
) But as Kim says, try it elsewhere to be sure (but I thought you got this pop in other places (rehearsals ??))
Do you have any power filtering ? a good idea if the wiring is very old.
Glad the tubes sorted the vol drop, easy fix
A/B box, so unless you go for something with some electronics in it, the Y cable will do the same thing as the A+B setting an A/B A+B box. If you replace your Y cable with this, you would choose A (say JMP1), or B (MP1) or A+B (JMP1 + MP1). As you go mono into each poweramp channel (JMP1 L and MP1 R), so neither unit in stereo. Where an A/B box would be handy is if you re-patched it all like a PA with a small mixer. Is there a reason you want the GSP1101 in front ? Seems it's there so you can mix it all together (as I said earlier, interesting patch).
So my suggestion is use 6 or 8 chan desk with at least 2 FX sends so the patch would be:
Guit > A/B A+B box > A > JMP1 > desk ch 1 & 2 (pan L/R) | > mixer out
> FX send 1 & 2 > GSP1101 loop rtn L/R > GSP line outs (or mixer outs) L/R > desk ch 5/6 | >>> mixer out L/R > amp
> B > MP1 > desk ch 3 & 4 (pan L/R) | > mixer out
Turn up FX send 1 on ch 1 & 3 and fx send 2 on ch 2 & 4 (so now FX will be in stereo (don't tun up FX send on ch 5 & 6 or GSP will feedback). And set GSP FXs to full wet (no dry sound)
This then will all be in stereo (and will blow you away with how good it sounds). You control the overall volume of both preamps and FX via the desk faders.
To slot back in your DBX eq (I'm assuming you only have 1 of these and you use L for JMP1 and R for MP1) and your sonic crapsuliser (does this add any value ??, it will add noise) feed them from the FX sends and rtns on the JMP1 and MP1 so: JMP1 send & MP1 send > DBX > Crapsuliser L JMP1 rtn R > MP1 rtn.