Explain my Slave Setup to Me

[Rich_D]

I am slaving my 5w mini-Plexi in an EVJ head into my 5150 combo. I built the rockstah diagram line-out box but I added a speaker out to go to a 1x12 for load. The line-out box has a 5k linear pot for signal to the effects return of the 5150. I assumed the post volume control of the 5150 would still be in play using the effects return, but it does nothing to the volume of the sound coming from the 5150. Can someone explain that to me? I think I understand why the tone controls aren't in effect but I thought the post volume controls controlled the power section of the amp.