If I were to do this, and I wanted to be able to shape the bass tone separately from the keyboard, I would use a Line 6 Helix product. It would offer tremendous flexibility and programming options, great bass sound, built in expression pedals on the larger models, independent processing paths for the bass and keyboard inputs,
I would then use either a single powered speaker or an 8 over a small sub depending on how much volume I needed.
This wouldn't be cheap, but it would be incredibly powerful. The larger Helix units have a full midi commander that could do anything on the keyboards. It would take some setting up, but once it was dialed in, it would be very easy to set up and operate.