Hi there.
I have just recently received my Kemper Rack and I'm really happy with it so far. I didn't have a whole lot of time to really dial in tones but really wanted to use it live as soon as possible. Last weekend I was playing two open air festivals and 2 hours before we met to pack up the van, I just went into our room to put the Kemper into my live rig and tweak it real quick. So I hooked it up to the FX return of a 5150 and 4x12 cab, tweaked for about an hour and it sounded great.
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I used the same setup live but didn't mic up the cab - I just used it for stage monitoring. I used the main out XLRs to go into the board directly and it sounded great through the stage monitor wedges and according to the FOH guy it also sounded great outside. My old setup was a POD X3 Live into the 5150 and while it sounded great through the amp, it always sounded really shrill and harsh through the DI outs and monitor wedges on stage. I've been using Line6 heads etc. for about 10 years and it took me a while to really make them sound great, but with the Kemper it practically took an hour and it was ready to go and I was sounding better than ever.
Long story short, the Kemper is a great unit. I can't wait to tweak even more, I'm sure the results will be nothing short of amazing. I'm also thinking of going the FRFR wedge route in the future, in addition to the amp/cab in the back. I have heard nothing but great things about those.