Yes, I've got one of the KB12s. I bought it when PMT were blowing them out for £220 about six months ago. Whether it's underpowered or not depends on what you expect from it or compare it to, I suppose. For me, it's a £200 combo amp which packs away quite small and is easily powerful enought for rehearsals. That's up against a loud guitarist and a drummer who thinks the only sure sign that he's been hitting hard enough is if he ends up in A&E for precautionary X rays. Actually, we've rehearsed in a couple of studios where I've had to turn down because the KB12 has set off worrying vibrations in the walls and ceiling. So I can't fault it as a rehearsal amp, even with a loud band.
I've only gigged it the once and that was outdoors. So probably not a really fair test. But I would say that it got a little lost in the mix under very testing conditions. The drummer kept asking me to turn up the volume as he couldn't hear me. But turning up past about 2 O'clock had it starting to fart out quite noticably. I'm not sure if that was the driver going past its excursion limit or the class D head starting to clip. But about 1 O'clock was about it's practical limit.
So I get the feeling (bearing out that previous post which I've also read) that the KB12 is probably quite sensitive to room dynamics and placement. In a relatively small room, well placed near a back wall, it could blow your fillings out. But get a badly shaped room, or less than ideal placement, and you could struggle to hear yourself. But that's a difficult thing to test without trying it for real. For what it's worth, I've started putting together a rig consisting of a 700 watt head and, so far, a 12 inch cab. All off the classifieds here. I'll probably add another 12" cab before too long as I know we will be doing more outdoor gigs next summer. And I don't think the KB12 will cut it.
Don't know if that helps any.
A