| Bass amps do make good substitutes for acoustic guitar amps if the tweeter is decent.
You should be able to shape your acoustic guitar sound by adjusting the electronic controls for your guitar pickup on the guitar itself. If you do not have those kinds of controls (or one with a limited eq and shaping capability) then you might consider an acoustic guitar processor to bring out acoustic guitar tone.
Keyboard amps are even better substitutes, IMO. | 
