Just to dig a little deeper....
There are 2 modeled versions each of the SVT & Mesa 400. The Ampeg B15 is the Porta-Bass (portaflex), and there's also a unique FAS Bass amp model.
Of course, the old Fender Bassman models can be used and set to not fart out (unless you want that) as they do in the real world.
As @Challenger2 mentioned, the other "guitar" amp models can be used effectively for various tone styles. I've also used the HiWatts and Marshalls for different flavored vintage tones.
Though I like to keep my presets lean, I've lately been splitting my signal Hi-pass/Lo-pass, with the high-passed path getting various types of OD & distortion. This is where using the guitar amp models can be a great alternative to using a drive pedal model, and using the 4 channels to select from various amps and/or gain levels. With the fundamental low end preserved in the lo-pass path, you can get really aggressive with overdriving an amp model without fear of losing bottom end, or having the low end cause unwanted/premature overdriving the input gain in undesirable ways.
Don't forget also the Drive block has the Darkglass model, which can be used on its own as a complete core bass tone without a Cab block. I'm looking forward to the SansAmp GED that is supposed to be getting its own model.
I agree, that the missing Acoustic, Aguilar, GK and SWR models, for starters, are disappointing. BUT...it is what it is. IMHO, there are a lot of features that help me get past these omissions. Thankfully, I'm an Ampeg guy but I'd definitely use the other models if they were there.