I did try guitar + octaver + bass modeling amps. It sucked.
I recommend using bass VSTi's with real bass samples (which of course you can play with your midi keyboard). To my knowledge, the best are:
Spectrasonics' Trilogy (the one I use)
It's got different bass types (electric, 4 string, 5 string, acoustic, synth) with different articulations for each (finger picking, mutes, slides, etc.). About all you need. Here are a couple of demos:
Demo 1
Demo 2
However this one has been discontinued since Spectrasonics released a new/greater version of this bass module called:
Trilian
Though I haven't tried it, it seems even cooler with a huge list of real bass sounds. More expensive though but worth it if you do some serious/professional bass tracking but don't have bass gear and/or a bassist. It's fucking sweet.
Check out their electric bass demonstration
here.
Yellow Tools' MAJESTIC:
I haven't tried this one. It's cheaper and the demos sound decent. It contains samples from basses such as:
Ibanez BTB 5-String
Fender Jazz Bass
Fender Precision
Scarbee's Bass modules:
Also cheaper, high quality sound, though perhaps a bit more inclined towards funk/fusion styles. I liked them nevertheless.
EDIT: Keep in mind that often, how you play/program the bass module and bass tracks is more important that what the module can do and how many sounds it's got. You can achieve some pretty realistic bass tracks.
