I used a Kemper in studio over 3 days actually. The producer who owned the studio also had several of the actual amps from his profiles there too. So I was able to play the Kemper directly against a '72 Blue Line SVT with 6146Bs, a '66 BF Bassman and a '68 Plexi with EL34s. The Kemper profiles were spot on and extremely accurate thru the same cabs I played the actual amps thru. The few times I have played a Stage 800 was in a demo room at Guitar Center, once just again a few days ago as I said. I was able to get the volume up plenty loud, gig levels. As I have stated, I don't like it at all. Its very sterile sounding, lacks any kind of warmth and presence of a tube amp and doesn't sound anything like the actual amps its trying to emulate IMO. The Blackstar U500 does a much better job of emulating tube amps IMO with its 6L6 and 6550 responses.
All of the Stage's presets have no low end and way too much highs, even when I tried to dial it in better. The presets that try to emulate any kind of OD or distortion sound terrible the higher you turn the volume up to. It just sounds like a huge digital mess. Nothing like a quality drive pedal going thru a nice tube amp. As I said in another post, the main problem is the same problem with the Mustang amps, terrible generic speakers with a flat response. They need to have that flat response because it has to emulate so many different sounds. Ive heard Mustang's with speaker swaps that sounded much better but it still doesn't compare to the actual amps its trying to model. The Kemper sounded fantastic with the bass amp profiles thru the Ampeg 8x10 and the guitar amp profiles thru the Marshall 1960AX with G12-25 Greenbacks. Its a night a day different from a Stage 800 or a Mustang which sounds like a toy in comparison. I have not had the opportunity to use any on stage and going FOH with them wouldn't prove anything anyways. With a good sound guy and a quality PA, you can go to FOH with a DI box like a SansAmp and sound great.
This all just my opinion and there is no point in continuing to talk about it. Some don't like digital modeling amps and some do, nothing wrong with either viewpoint. I only eloborated on why I didn't like the Stage 800 earlier because someone asked me to explain what I meant when I said it sounded digital, so I did.