Smoothness through the mids, along with the top end power and extension you'd need for keys, in a cab that can handle bass guitar without going into premature fartout, tends to be a rather boo-teeky combination of characteristics. Electric piano in particular is quite demanding in terms of sound quality. I think what you're looking for is out there, but you may have to expand your search beyond Guitar Center. The Acme B4, fEARless 212 by Greenboy, and BigE 110 or 210 by Mike Arnopol come to mind. Or maybe something custom by Low Down Sound, with custom crossover by Ralf Patterson.
You mentioned looking for a 410 with "good flat response". If tasked with buidling a cab for bass + synth + electric piano, I would not go with a 410 format because the horizontal dispersion will not be very good. There's a reason PA cabs look like they do, and it has to do with getting good, uniform horizontal dispersion. Imo that's even more important than "flat response", because you can (to a certain extent) improve the frequency response with EQ, but you cannot do anything about the radiation pattern with EQ.