This on board amp from Parts Express looks good, at a price that makes sense.
https://www.parts-express.com/dayton-au ... h--300-798
It could be mounted on just about any of our full range cabs with minor modifications. It has enough power to drive a pair of cabs, so it would be easy enough to install a 4 pole Speakon on the cab containing the amp and one on the extension speaker. Full DSP that includes crossover, EQ and limiting plus Bluetooth makes this a very attractive rig.