I love the Kappas as absolutely nothing beats them on price except the subs, nothing. The Polk Audio MM series subs have higher SQ and better specs, albeit sealed. I've never been a fan of ported, only to point to the Rockford Fosgate P2s, but the P2s have lower specs across the board compared to these 120.9w. The kappas should do extremely nicely and the only sub better in that scenario is going to cost $500+ each easily.
I haven't been a fan of Sony/JVC/Pioneer (entry level) amplifiers since well, ever actually. I knew friends that bought them growing up and they immediately replaced them. My first amps I owned and installed were Alpine, Rockford Fosgate Punch and PPI (when they were still good). Manufactures at entry level leave out components or use cheaper quality to achieve these price levels that frankly, I could never justify. The list of potential problems are endless, lack of power, faulty power rails, fried components, potential line noise to name a few and I would rather purchase a quality amp the first time. If you need something and can't wait then I suppose it will have to do though at a minimum I would suggest a high quality 5-channel from alpine, JL, RF-Punch through ebay, craigslist, diyaudio.com, etc.
Off the shelf, the "lowest quality" single amplifier (5-channel) I'll recommend for our vehicle is the Alpine PDX-V9. There is absolutely nothing that comes close to the sound quality of Alpine, power availability for it's street price of $425. If you can find a nice one used, I would pay no more than $300 for it. Comparing amps in the used market is a grey area I simply don't enter as there are too many variables. Just make sure it's a quality amp, buy local or with a guarantee that it works and you should be fine.
Think of your amplifier as the engine powering your
Saturn V Rocket, one simply doesn't install a tractor motor when you can have the
Rocketdyne F-1 for relatively the same price.