So, I had to borrow a friends Dual Rec for my gig last night, as my S1 is going back for repair soon. I took my Decimator G-string with me and my new TS9, and my signal went
Guitar > TS9 > ISP Guitar In > Guitar Out > Amp
FX Send > Dec In > Dec Out > FX Return
The FX loop was engaged, and no matter how high I set the threshold of the Decimator, it would NOT get rid of hiss or any feedback. I set the Send and Mix controls both to full tilt.
WTF was I doing wrong?
Set the send level all the way up and the return level about halfway and your set.
The mesa has a parallel fx loop not the normal serial fx loop.
So setting the send all the way up makes it act like a serial loop. Which is what a ISP needs.