Thanks for the thread resurrection, I have an update!
Update: I've been using VTRM/RPM in parallel and love it. Here's a live soundclip from the wee hours of 1/1/14. I think I've dialed in my tone even better since then, but I'm happy with this (clams excepted):
https://soundcloud.com/fuzzbass-4/sets/hightest_nye_2013-2014
I haven't yet tried VTRM/RBI in parallel, but while I think I'd like it, I still suspect that the RPM with its sweep mid is a better match: the VTRM can get plenty bright. That said, I still think RBI/RPM is an excellent match.
Yep, that's one of the advantages (a huge one) to running these SansAmps in parallel. For a little while I ran a true stereo rig: using a stereo power amp, I ran dirty signal into one 2x10, clean signal into the other. But then I realized that I couldn't fully appreciate the blended tone unless I was standing away from my rig or kneeling in front of it, so I've gone back to "faux stereo".
I have to add that the Blend control on a single SansAmp pre (any of them) allows you to balance the SansAmp tube emulation with the clean tone of your bass... so it's sort of like running clean+dirty... but no, that's not nearly as powerful as running two independent channels.