Most of the 10" cabinets around are designed for bass players though there are some guitar cabinets as well. Looking around online I see there are some monitors and PA cabinets that have a single 10 paired with a high frequency horn. It maybe worth considering a couple of those with a power amp to get a full range sound that would work for keyboard onstage.

The 10" Kustom floor monitors I was looking at were pretty cheap at $75 each and had a built in cross over for the high frequency horn. If you used 4 of these you could also put one on the other side of the stage to get good stage sound coverage.

Just a thought anyway. Still I think getting more smaller speakers is the way to go.