I just installed 8 MMC650s in my boat. They sound great. I was able to pick mine up for 99.00 a pair. The main difference is the MMC650 comes with a crossover and function as a component set as opposed to a coaxial speaker. This is why you see a difference in price.
I run them in parallel off of a 4 channel 360.4 Zapco amp. Louder than I could even imagine. Gains are turned way down, but Zapco's are very efficient amps, making them appear to be underrated in power.
I just put the db650's in my boat and they sound wonderful very crisp and distortion free. I'm very pleased with them and to be honest the bass reproduction from them isn't to bad either. I got them at cardomain.com for 56.99 a pair and at that price the quality is tough to beat.