Hey Jazzbass19, I’m no expert, let’s get that out of the way…so if the ‘Gain" is the level control of the slave amp’s preamp section, then yes, I believe so…if the input to the slave is the FX return, and NOT the slave amp’s Input jack, then we circumvent the slave amp’s preamp section…more modern amps - with FX loops or ‘send’ and return’ for effects, allow this relatively easily, whereas if the amp is older and lacks the FX loop, it gets more complicated…I found this article from Seymour Duncan, and I think it’s what I’m after, although they are discussing guitar amps, the principle should hold for bass amps too…