Hey RG, the issue is mixing inst level and line level. The silencer is either, not both at the same time. The silencer input is fed to its send, so if the input is inst level (your guitar), that's what will go out the send, if the silencer input is loop send (GX700 or rock rockmaster), that will be the silencer send level (line albeit dialed down via send level control). The manual diagram puts a noisy distortion pedal in the silencer loop which is inst level in and out. If you put the rockmaster in this loop instead, you create a (large) gain mismatch because it's inst level in and line level out, the rockmaster isn't designed to go in a loop or fx chain (e.g. pedal board) where the noisy pedal is. Obviously you can make it work by adjusting the various send and return levels etc. but it's like you have a gain volcano with a plug in it, so the potential gain is still there in the circuit i.e. squeal city. You can get some interesting/amazing sounds this way but they are always on the edge. One of the best guitar sounds I ever created (back when I was 14(ish)) was:
Guitar -> battery powered microphone preamp -> old 5w tube radiogram crystal input (before magnetic LP PU cartridges), 5w speaker out -> 100w amp input (it normally wants 1v (not 5w
)), 100w out -> 2 x 20w twin cone hifi speakers. So massive gain mismatches all over the shop. Sounded absolutely awesome until I started blowing my dads hifi speakers
(he was not impressed). Also squealed like a banshee but that's partly how I learned to play.
Another example more similar to your gain mismatch is when I used a Rockman for solo boost:
Guitar -> a/b a+b switch, a to Marshall 50 input, b to Rockman input, Rockman headphone outs, left to Marshall 50 bass ch input and right to Marshall 100w Artist input. I put resistors in the Rockman output leads to bring them back to inst level for the Marshall inputs. This sounded great but squealed uncontrollably when I stopped playing (because of the Rockman gain mismatch), so I had to be quick on the a/b switch to turn the Rockman input off at the end of songs, the singer called it the beast LoL. So even though I'd balanced the levels (resistors in the Rockman output leads), the potential gain in the Rockman just went off, same thing is happening with your rockmaster.
As we discussed ages ago, to use the 2 preamps, you could split the guitar signal so both preamps get inst level in and bring it all together with a small mixer. But then you don't get to load up/boost the rockmaster input from the GX700 preamp stage (which IIRC you like to) or get the GX700 fxs on the rockmaster. So since the way you do it gets the sound you want, you need to control the gain mismatch (squeal potential). So running the rockmaster in the silencer loop will (and does) help. A normal gate after the guitar will also help because the whole preamp setup wants to feedback. Also using more sophisticated gate(s) could assist as they give control over gate attack and release times, not just threshold.
I like to rehearse as similar to stage as I can, so cab(s) behind me. Unless I put the cabs up to ear(ish) level or get a fair way from them, I don't hear the top end as much as it all goes into my legs. Consequently my patches probably have too much top end.