Obviously I LOVE the 9.0. It's become my absolute go-to, to the extent that I'm thinking of buying another just as a backup.
Following up on some of Heath's comments...since I own both the 6.0 and the 9.0, hopefully I can add a bit of reinforcement here. The 6.0 is a terrific amp, IMO...when used with Shuttle cabs. Paired with one - or two - 12" Shuttle cab(s), I absolutely love it. Great sound, amazing portability, and plenty of power...for the right environment. I use it for practicing, rehearsals, and small bistro/restaurant gigs. I have used it on much larger gigs, and it still sounds great - especially running both cabs. Plenty of lows, ample headroom, and a mid-happy sound that really cuts through the mix and presents the character of the instrument running into it.
Dislikes with the 6.0.... I don't care for this amp driving any other cabs. Epifani 10's, Aguilar 112, and Bergantino 112...just doesn't do it for me. You can certainly EQ in some of those missing lows, but I don't like the end result...just not natural sounding, for lack of a better description. I also don't like the 6.0 with my more aggressive instruments - namely my Sadowsky M5 and MTD. Very sterile sounding, IMO, with a little too much 'clack' and not enough 'oomph'.
The 9.0, on the other hand? It just works. Those same cabs mentioned above...bada boom. Every instrument sounds great through this amp. Granted, I'm not trying to drive huge cabs, but I've played some pretty big rooms and outdoor events with this amp and it doesn't feel as if it's even broken a sweat. I just giggle at the sound, especially after load-in. Something this small and light shouldn't be able to do so much.