MT15 huge level boost from fx loop

[HardBoiled]

So I got a new noise gate pedal (Klirrton Schnauze, it's pretty great).
When I use it with 4 cable method it's all fine, but when I use it 4 cable method with a preamp pedal into the fx return, there's a huge level boost. (guitar > schnauze input > preamp pedal > schnauze loop input > MT15 fx return)

I like using the amp's power amp for different pedals and the schnauze works just fine, it's just the level boost that's weird. and it occurs only when the gate is engaged.

I got a temporary fix, but a better understanding of the fx loop and finding a permanent solution would be much preferable.

Anyone has any idea on what's going on?

 

[Boogie]

Line level matching. That’s why some studio FX have a -6dB (or something close) pad option to help match levels. Not all boxes work with all amp loops, and since the MT doesn’t have a send/return level adjustment, I’d look for a different pedal for this amp. It’s all about gain balancing.

 

[HardBoiled]

But doing the same thing with my Orange OR15 there are no issues. It's just the MT15's fx loop that's having the issue. And everything in the chain is instrument level, is the MT15's loop line level? Why would it be?