I was running AMP Out straight into the front of the Carvin Vintage Tube 33 with really good results .... even being in Nashville using some well schooled engineers, I was getting compliments on the tones I was getting ... and that amp just doesn't really have GREAT tone to start with ....lol ...
Make sure when you are running into the front end of an amp, that you have consulted the amp manufacturer specs to find where the tone knobs are basically at "0" ... if you start twisting them away from their neutral settings you are basically 'double pre-amping" and it always seems that any minor adjustments are actually multiplied ten fold .....
You may need to tweak the speaker selections and mic placement parameters, because you ARE getting some amp simulation on TOP of the actual tone of your cab .. so, it can be a little touch and go ..... maybe I just got lucky as quick as I did ...
For me, for a fly rig, I'm not looking back with any buyers remorse for this purchase ... extremely happy for what I use it for!!!