NPD: EHX Silencer (4 cable method troubleshooting anyone?)

[eclecticsynergy]

Amp effect loops vary in level. Some are hot enough to drive the power stage of the amp.
Unless your preamp is super noisy, probably best to use the pedal's send/return as intended: with effects in its loop, not a preamp.
If the input of the gate pedal is seeing instrument level and its return jack receives line level, that could conceivably cause problems.

 

[PFDarkside]

If it works with pedals but not the amp’a preamp, he’s got the signal routing correct.

I think you’ve got an impedance, signal level or ground issue between the front of the amp and the loop. The best workaround is to use the input at the front of the chain, pedals in the loop and the output to the front of the amp. If you are replacing the amp long term, just deal with it like this and if not investigate the ISP or a rack gate that is designed for line level signals.