I have all three of them. A short summary:
VT Bass DI: Most useful of the Sansamp series for bass imo, thanks to the "Bite"-switch which afaik is always on on the other VT Bass versions. Works nicely as an amp sim and can also be used as an overdrive. The character is well known and quite close to an SVT-style rig plus 810. The overdrive remindes me of a pushed 810. I use it for practice/going direct if I have to (best "speaker sim" so far, still GASing for a Torped cab sim...) & my main rig is a Stingray or Squier CV P through an Ampeg SVT IIp -> Crown XLS -> Ampeg Heritage 810.
Darkglass B7K: Modern sounding distortion, I really dig both the voicing as well as the flexibility it delivers me. Although I really hate blend knobs on bass distortion/ODs, this one is usable to add in a little clarity. Not that the distortion part is loosing lots of low end...I toyed around with it shortly on a clean setting. It can do that and the EQ works nicely there as well. It will remain the modern voicing though, never tried to bend it towards a more "vintage" sound. It might be able though. I used it as my main distortion, since my "clean" sound is quite dark I used the EQ extensively.
Tronographic Rusty Box: The Rusty Box is also modern sounding imo, and with the knobs at noon it has less bass than the VT Bass DI. The EQ does work nicely though, especially when using a real cab. It seemd there the EQ made bigger differences than when going direct or using a cab sim. The character of the distortion reminds me more of the Darkglass than the VT Bass DI and you can easily get some tones out of it that are similar to the ones you get from the B7K. I haven't used it live yet, just wanted to get it off of my GAS list.
We plan on doing some recordings this summer, I might be able to do some proper (Pre -> Cab -> Mic) ones of the Rusty Box then. If I find the time during the next couple of weeks I can do some going direct. Especially on the more distorted tones a cab (sim) is necessary imo.