Your mention of power-tube breakup and touch-sensitivity makes me wonder what you would think of the SL900.
The Streamliner's biggest strength when it comes to breakup, is that it separates the output level, distortion and sag characteristics- at one extreme you can get true tube overdrive at low volumes (like a master volume amp), and at the other, you can use the tremendous output power to get clean tones at louder-than-SVT levels. At higher volumes, the dynamic feel is extremely "tubey", as well. The ADPM circuit gives you just enough sag and give.
However, this is very much IMO and not IME because I never use distortion on bass, but I still see a couple of potential snags for a diehard SVT fan:
One, is that to my ear, the overdrive characteristics aren't quite as complex and reactive as a good tube power stage, more like preamp distortion or an overdrive pedal. YMMV, and all that.
The other, is that the Streamliner doesn't attempt to be an SVT clone (or a Mesa 400+, for that matter). The SL is it's own animal tone-wise, and for my tastes, that's perfect- I kind of like SVT clean tones, but I LOVE my Streamliner.
So, knowing how personal peoples tastes are when it comes to breakup characteristics, my gut feel is that you should probably audition a Streamliner before you sell your SVT, even if it means a couple of hours drive.
PS: Other people are getting double-duty out of their Streamliners, by running other preamps into the power amp input. I suspect that a Tech 21 VT or VT Deluxe could turn an SL900 into a pretty sensational SVT modeler...