okay, here's my intial opinion.
SVT7-Pro (w/stock JJ 83S/12AX7)-compared to the Streamliner 900 (with 3 NOS RCA 7025 (12AX7's):
they seem to have the exact same output capability (volume when cranked).
the 7-Pro can actually dial in a little more dirt if you want it
the 7-Pro is cleaner, clearer,more transparent and 'natural' sounding- so i feel alittle more 'connected' to it (BTW, it's also actually brighter than either of my SVT's)
the 7-Pro- has more harmonics and sustained notes often go to the octaves and thirds when cranked- almost too much. (even my SVT's don't do that very much).
the 7-Pro has more eq flexibility
it's super punchy and full sounding without any weird harsh peaks or cloudiness.
the Streamliner 900:
all of the notes have a bit more girth than the 7-Pro, especially higher up the neck, and has a little more inherent punch than the 7-pro
it definitely has a tone/feel of it's own.
it reminds me a little more of a 100% tube amp but it can get 'cloudy' if you don't have its eq set properly.
it's not quite as 'touch sensitive' and transparent as the 7-Pro.
overall, i wouldn't have a problem or complaint gigging with either one of them. IMO, they are both great heads. i haven't dragged out the SVT's yet to directly compared them, but i did go back and forth between the STM900 and the 7-Pro. neither one sounds and feels exactly like my SVT's (at least not yet), and in some ways the 7-pro reminds me more of my SVT's but in other ways the STM900 does.
if you're looking for a powerful, lightweight substitute for an SVT, i think either one of these would do it for you. right now, i'm slightly preferring the 7-pro, but then right after that i plug into the 900 and think it's equally as good, and i'm super picky about my amps.