Not wishing to derail - but i guess im not as the OP got what he wanted - but im in a similar position too.
I just picked up my first valve amp:
100w of british-made valve goodness
Untill i save up the dosh to get a Matamp GT200, this will likely be my main practice amp with one band and ideally my gigging amp with both bands im in.
It will only run 8 or 16 ohms. TaySte (who I bought the amp off) supplied me with a y-cable that should allow me to run 2 8-ohm cabs at 16ohms. Its a practical solution but just seems to be a bit messy and really I'd be happier with a single 8-ohm cab, also the cable is Two speakson jacks into one 1/4 inch, my new SWR is only 1/4inch.
I've taken the head to practice with one of my bands and put it through a Aggie GS112nt and the tone was great but it looked really silly and was pretty much shooting valvey goodness all over my feet (urm, sorry, that was crude). The Aggie is a keeper and lives at my practice room where it is about to be joined by my Ampeg Micro VR.
So I got hold of the SWR 2x12 you can see in the pic. so far, so good. so now i've got 2 12inch speakers pumping out bass as opposed to one but im still curious:
I've seen alot of people with lower-powered tube amps go for 2x15s. My only main concern is that come that 'big gig' situation, that I wont really hear my amp and dont want to run it at full tilt for a whole gig coz its stocked full of the Brimars and Mullards that it was built with and dont want to red-line the damn thing just for the sake of being heard.
At the practice session mentioned above I was only runnin one channel (you can daisy chain them) and only into a single 12incher, but volume and master at about 7 each. It was over driven without being super-distorted but really I just wanted some opinions as to whether a 2x15 would project more volume?
sorry if these are n00b questions, i'm just startin to venture to the amps forum now my effects GAS has largely died.