I would say stick with a 12-inch or 15-inch two-way cabinet. Self-powered or combo is the way to go. Anything smaller does not do justice to the mids and lows of a good keyboard, especially the piano and organ voices. Presently, I use a Bose L1 Compact, but I am doing OMB jobs for under 70 people. The bigger L1 with one or two subs is fantastic if you have the $$$.

I have always been partial to EV and reinforce Uncle Dave's suggestion for a powered sub. You can pick up one of these for under $500 and it is useful in so many situations. Use it when you need it. I find the Roland KC amps to be harsh/hard, prefer the larger Peavey keyboard amps and would seriously consider a powered 2-way cabinet from any of the trusted brands.

Nigel...I use a US Fender 410, running my Triton and an SP200, via a Behringer 602A to balance. Works for me. I also have a couple of Peavey KB100's (housing a 15" BW) but the 410 does the job nicely. I prefer it.

Yep, I did use a powered PA cabinet for quite a while with a 15" and a hi freq tweeter but I never was happy with the high mid range frequencies. When I got the Traynor with a 12" 2 x 8" and a tweeter I was much more happy about the sound.

Playing in a live rock band with a bass player the hi mids are much more important to me than the lo frequencies. Though if I was in an OMB situation that may be a lot different.

Though the bass player in my band uses a 4X10 cabinet that provides both a great low end as well as crisp highs.