Hughes & Kettner sound

I have not played any of the quantum stuff, other than their cabs. I do, however, use a HK Bassbase 600 head and older BR series HK cabs. I'm currently playing R&B/disco/funk and I don't think I could get a better tone than I have now.
I did not like the Quantum series cabs. I like the high end sizzle of the tweeter and the Quantum concentric tweeter thing didn't really do it for me.

 

From the pictures I saw online of the quantum combo, it appears as though it has the concentric tweeters, which is what the 210 cab i owned also had. This is not like a conventional tweeter that you can turn off; it is more like a home stereo speaker in which the tweeter is part of the inner cone of the driver itself. they call it dual-cone technology. i believe eminence makes the dual cone neo speaker and you can view it on their website. As such, you cannot turn it off, but I doubt you would want to. The smaller inner cone acts as a tweeter in a sense that brings out the higher frequencies. My only complaint was that after using cabs with tweeters over the years, the quantum was not as bright as a conventional tweeter. You could certainly use the tweeter on your second cab for the high end sizzle if you wanted.
On my bassbase600, the more gain you give it allows you to achieve tube saturation, as they call it, which gives you the slightly overdriven tube sound, which I assume is what the tube growl knob does on the quantum.
I'd certainly like to try one out. HK makes fantastic amps. If you're worried about the HK tone being too "rock," keep in mind that Larry Graham, Bootsy Collins and Norwood Fisher of Fishbone all used HK. I think Norwood is now using all warwick stuff, but I believe the other two are still HK