I agree with UD's comments above about just adding a Sub-Bass to the keyboard. Unfortunately most of the smaller sub-bass are nothing more than a boom box producing upper bass only with no low frequencies at all.
I once tried an expensive powered Sub-Bass (the name escapes me, but German I think) This produced just very low Bass only and added that bottom end feel, which once you hear (feel)is difficult to live without.
I wanted to add my input about speakers because it can be wrongly assumed that just adding any old Sub-Bass will greatly improve the keyboard sound....go for quality...not quantity.

Graham UK