I haven't had the k3000fx since I got the HD500 and I had a Peavey KB4 between them. The k3000fx sounded better than the KB4 and the KC550 sounded MUCH better than the KB4. I now have a DT25 mini stack.
The way I would describe using the KC550 is like the sound of an amp mic'd up going through a really nice PA. It comes closer than any of the others to soundling like a guitar amp and works really well as a replacement for a guitar amp, but it doesn't have the same feel as a real tube guitar amp.
I haven't yet tried the new bass amp with my DT25, but I was shocked at how good the acoustic models on my JTV sound through it. It should even be better with the new preamp model on the HD500 and the new acoustic models on the JTV.
Anyway, I would still highly recommend the KC550. It sounds great for electric, acoustic and bass plus you can use it as your entire PA if you ever do an acoustic solo act, which I still use it for. Only problem I had was the size and weight. I just got sick of lugging it around. That being said it is smaller and lighter than the KB4 and it has wheels unlike the k3000fx.