I presume that weird Kustom cable bypasses the Volume & EQ on the 'slave' amps? That's how the FX Send to FX Return works on Rumble amps, which is great for stacking 1st on top of 2nd and (possibly) blocking the bottom 2nd amp's controls. If so, that weird Kustom cable may facilitate some way of propagating the first amp's pre-amp signal to all the subsequent 'slave' amps?
But going from FX Send to the Input of the 2nd Rumble amps doesn't 'enslave' them! The Volume & EQ controls of the 2nd, (and 3rd, and 4th, ...) amp are still active and need to be adjusted separately! For wet/dry applications, and others, that is what you want. Otherwise, it may be a little annoying to make sure the 2nd, and other, amps are EQ'd flat and Volume set where needed.
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That answers the question I had a couple of days ago concerning hooking up two combos but allowing each preamp to be used. Thanks, 007!