I like an amplifier @90 db on up. This would have been a great choice for the money and compact sized for your install. THD is pretty nice on it, has Low pass and High Pass Xovers on it. Ive ran a similar smaller NVX of which is a little more expensive than this one. But for the size and RMS output for the speakers you have, you will still have a little head room to spare.They run cool and dont over heat.
Kicker has gone down hill since that early 2000s in my experience on speakers. The sound quality just isnt there/Sound hollow and Muddy compared to the Polks I mentioned, and the Kickers do not go o the dee response like Polks for under $100 for a pair of 6.5s Coax. Thats me and my experiences.Id put the pols up front and the Kickers in the rears somewhere, Especially since you already have them. Use what you got. No sense spending ay more $. But get that Sub in an enclosure for sure. Its in a poor environment and not serving any justice