Two words: Line Level.
The FX loop is line level and 95% of the effects pedals out there need a -15 dbu PAD in front of them to use the Randall Loops. The PAD works essentially like turning your channel volume way down. Framus also uses line level.
EHX stuff is designed for -10 dbu unless otherwise stated.
Eventide Timefactor, Pitchfactor, and some of the TC Nova stuff is for Line Level.
The FX loop is designed for pro rack gear. If you don't want to go that route, your best bet is the TC Nova System, or if you've got the bucks the G-system.