Hi. I'm new to the Eventide boxes too, but I'm running a TimeFactor and a ModFactor through the effects loops on my Mesa RoadKing. I'm running the ModFactor through the serial loop and the TimeFactor through the parallel loop and getting good sound from both (as well as being able to switch them out completely with the amp king kontroller footswitch). Two things I'd suggest to check/try – make sure you have the two switches on the back of the stompbox (InLvl and OutLvl) set to Line (they would have been set to Guitar and Amp out of the box and when you used them in front of the amp). If thats ok I'd suggest try changing the KillDry setting – that may be causing some issues. If no joy there, check your manual on the procedure for viewing the input line level and make sure that is not excessive. Again check the manual for how to adjust the output level and try reducing that level in case its boosting the signal too much into the power amp stage and causing feedback. Give that a go – would like to hear back if any of those help.