I guess it's fair to say it's not all in the amp, but Ashdowns generally give me a warmer tone from what I've heard and tried out.
I know all about Ashdown 900, but it's what the dude who's lending it to me has, so yea, can't complain
Will be only using 500W (575W) with my GK NEO 412 out of it, but it should be enough.
700RB is a great workhorse head, and sounds great, but it has a bit of a low mid scoop, its low mids are very aggressive and tighter (not as fat) and it has abundance of high mids and highs (which I'm not into). TH500 is a great head too, I've had the Tone Hammer preamp.