I think a sub woofer would give you more problems with the bass competing with the bass guitar. If you were using your keyboard as an arranger in a OMB situation then I would definately go that way. But playing keyboard in a live band is a totally different thing. Roland and Behringer make keyboard amps with 12 inch speakers that you could try out in stores around where you live. I definately wouldn't go with anything bigger than a 12 inch speaker. 15 inch speakers will definately give you the bass problem you are trying to avoid.

I think 10 inch speakers would even be better than 12 inch. They will produce high frequencies well without the sharpness you are getting from having most of your high frequencies being produced by the tweeters.

I would seriously think about a cabinet with 2 or 4 x 10 inch speakers and a good quality power amp. They can still produce bass because you have more speakers pushing air but not as much in the very low frequency range which sounds like is your current problem. It would be nice to have high frequency tweeters as well as the 10s but the 10s will produce mid to high frequencies pretty well without the sharpness.