Mark II fx loop and Boss pedals?

Wondering why some stompbox effects sound just fine in the FX loop of my Mark IIB and Mark IIC+, and others don't?

Specifically, a Boss CE-5 chorus, PH-1r, PH-2 phasers, and a Malekko Echo 600, all sound fine and dandy through the FX loops of these amps, while others, such as a RV-3 Reverb, CE-3 Compressor, DS-1 distortion, and a tremolo I built from a BYOC kit, all exhibit either very weak volume (the compressor and distortion) or weak volume AND distortion, almost like a bad patch cable (side note -- I confirmed this wasn't just a matter of bad cabling).

Yes I know that Mesa recommends only line-level rack-type gear in the fx loop and stompboxes out front.

I'm wondering what could account for the differences?


Side note: If I play my guitar directly into the DS-1, directly into the FX Return, it actually sounds pretty good.

Most stomp boxes are designed for high impedance / lower instrument level signals, like the signal from your guitar. Most FX loops (especially Mesa) are sending a much stronger low impedance (line level) signal through the FX loop.

You are overdriving the input of your pedals with the hot signal level of the FX loop.

I guess what I'm wondering is why the line-level signal from the FX loop affects various pedals differently. Some of the Boss pedals sound just great in the loop, even some older ones like the PH-1r. But others don't, and that's interesting to me because I would have assumed that they would have similar input circuits.