My experience with these devices is they work best plugged into something other than a guitar amp. I got very frustrated with these until I fully realized this. The trouble is that the amp simulation will be in conflict with the sonic characteristics of your amp. No matter how much fiddling I did with my amp to make it "flat", it just didn't sound good.

If your amp has an effects loop, you can plug into the in of that for better results since this will bypass the preamp in your amp. Take care to have the volume turned down on your device since that is now what controls your volume!! Another alternative is to get a powered PA speaker and connect to that.

I've used the Line 6 POD, the Zoom G5, the Boss GT-100, and the Eleven Rack. All of them sounded unsatisfactory to me when plugged into a guitar amp but sound great when plugged straight into my computer or a PA system.

On something like the Zoom G5 or Boss GT-100, you can disable the preamp and only use simulated pedals...tubescreamer, wah, distortion, etc. That will work plugged into the front of your amp since you are using it to emulate effects pedals...not an amp.