I own both of these amps, so hopefully my info will help. The RKII is definitely an awesome amp. The cleans are astounding, low gain rhythm sounds are perfect, and ch.3 and 4 sound great. One thing you should know about the RKII and indeed all rectos in general, is that they tend to "growl", when compared to a MKIV, or any amp with a very tight, focused gain signal. Rectos have this huge, sagging bottom end and a massive, complex mid-range voicing that makes it sound like there's a fuckin' monster inside your cab that's screaming at the top of his lungs. It just has to be heard to be believed, you can't really describe it. It just sounds like total destruction. The MKIV is huge sounding, but in a different way. Everything about this amp is much more focused and tight, especially the bottom end. It has a sizzling top end that makes everything sound like it's on fire. There isn't really any mid-range growl going on here at all. I'm sure you could dial it in, but it's not what the MKIV is for, and the recto does it better, anyway. The tracking on the MKIV is so precise that when you play fast, it almost sounds like the amp is playing ahead of you.
If you were to compare these amps to fighters (don't ask haha), I would say the RKII is someone who just comes in and smashes the hell out of everything with no rhyme or reason to its methods, where as the MKIV is very precise, and hits where it hurts.
That's why I play through both! Having both of these bitches cranked up is sonic bliss :twisted: