If i had to use my amp for keyboards and bass id probably put a little mixer between my pre amp and my power amp.
I used to use a Morley aby and boss ls2 for switching between 2 instruments. Lately though i just mute and switch.
If you want something small and don't need a ton of volume you can use a little sans amp pedal as a pre and run everything into a "big" keyboard amp. A lot of keyboard amps are built like compact PAs the peavey kb5 for example can handle bass pretty well especially if your playing a 4 string. They are becoming a popular amp for drummers who use vdrums also