If you are looking for a good clean sounding amp rig for live use, as opposed to something for heavily coloring the sound when recording (like a guitar amp), I recommend Carvin for best bang-for-the-buck.
Carvin.com - Guitars, Amplifiers & Pro Audio
I did a lot of live gigs with a multi-keyboard rig in the L.A. rock club scene for many years and I had great results with a pair of LSx1503 three-way cabinets and a pair of LSx1801 subwoofers (actually, the model numbers were different and the design was slightly different, these current models seem a little better than the versions I was using).
Carvin.com - Guitars, Amplifiers & Pro Audio
I had a pair of large stereo power amps to run my rig (1800W RMS), but these new speaker models are also available in self-powered versions.
If you need something a bit bigger or smaller than these, their other models are also a good deal.