Tried all this last night, was absolutely marvelous.
The 4x10's are much much lighter than mine and kick out some serious bass, they have a beautiful even response no matter what I played, for the weight and cost these are some serious contenders they are just stunning and will look even better with the metal grills instead of the fabric ones.
2x12 well I was amazed, Monz had been saying how hifi it sounded and this is true but you can dial that out and get it thump as anything, it really did range from sounding like a 2x15 to a hifi studio monitor and this is without a tweeter, simple stunning I'll be ordering one as soon as my Buster sells.
GT200, a great head, I knew this already I've played through them lots but they never cease to amaze, plenty of clout and more power than most people would ever need, the drive on it is still the best distortion I've ever heard be it pedal, rack or amp, I still can't afford one though but one day. Great amp and for the price it sits nicely in other high class amps and because it's so good thats why you never see them come up for sale second hand
Also it was quite funny being in a room with 5 tube amps and 22 10 inch speakers and then 18 12 inch speakers including what my guitarists had lying around, it was a shame my 400+ is in for a service as I could have powered all the bass cabs from that and my Strategy and really heard what it had
Congrats Glyn you've got a stellar amp, I hope your gig with the 2x12 sucks and you have to give me a call