Lets see now--Dave sings, Donny sings, I sing, and nearly every keyboard player I've met in the past 20 years sings and nearly all prefer the sounds coming from a 15-inch speaker over the sounds coming from a 12-inch speaker. Some of the bigger bands, such as The Hubcaps, have much larger speakers, but I've never seen them using a 12-inch anything. The last time I performed using 12-inch speakers, even after a lot of EQ work, I was not at all happy with the quality of both the keyboard and the vocals. During the second set I switched to the 15's and everything sounded considerably better. My advice is: If you're going to do this stuff for a living, then put the best sound out to your audience you possibly can produce. If you want good, solid bass, you'll have to move lots of air. There's no question that you can move a lot more air with a 15-inch speaker than you can move with a 12-incher. Just MHO!
Good Luck,
Gary
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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)