I seem to be almost exclusively now using the KPA as a speaker sim and effects block with my real tube amp (via THD hotplate line out and KPA amp section "off")....which is great ! You get the real "roar" of saturated tubes and the true touch sensitivity of a real amp and some great cabs from the KPA.
I have tried to like the KPA amps section, but I just don't. Not having a true tonestack is a big problem for me to craft the juicy overdriven sounds that I use, plus the KPA amps are not as touch sensitive as a real amp. I know a lot of you think they are, but if you profile your own amp and then compare the true differences in touch sensitivity in playing (not just testing sounds) there is a big difference. On very high gain sounds of course you won't notice this, but using "just breaking up" sounds and medium gain sounds that have a natural decay to them there is a very big difference, and the only way to get the same fluidity with the KPA amps is to use a compressor, but then that changes the sound of the amp so I don't want to do that.
Having said all that, apart from the software unreliability of the unit I am very happy using it in my studio to provide excellent speaker sims to my real tube amps....it gives me the best of both worlds, plus of course I can profile my own real cabs easily too.