I've been integrating some analog pedals in to the KPA Stereo FX Loop, but was getting distortion on both my MXR Analog Chorus and MXR Uni-Vibe (current model). It was making high end static crackles which was the FX send signal overloading the pedal. Easy to tell since Loop volume did nothing to alleviate the problem.
Also, my Line 6 M9 digital effect is perfectly happy with the FX loop, so this wouldn't apply.
Here's how I solved it -
I went to to Amplifier section in the Stack section. Press Page until you get to the end. Adjust volume -5db, or whatever makes the crackling disappear. Exit back to the main browser. Adjust Rig Volume +5db, or whatever you cut in the amp section. Worked perfectly and doesn't seem to affect the tone. I've only used this fix on a clean profile, so far.
Hopefully someone with more experience will tell me if I'm doing it wrong, but it worked for me. Not sure if this would work in the Stomps section, I generally put those pedals in front anyway.
BTW, I tried attenuating the analog signal at first. Probably because my brain was already operating in an analog mindset. Several problems with that approach. I tried using a volume pedal, but it drastically changed the tone (as expected). I used an inline attenuator, but it changed the tone and cut the volume way too much. The other issue was turning the Effects loop off and getting a massive volume spike. Anyhow, I hope this helps a fellow noobie like myself!