If I might add in some subwoofer insight.
The Kicker comp is a decent well made "budget" subwoofer. You can get excellent performance out of them.
Now as Ranger has had to deal with me on this stuff it may be a repeat of the same.
The performance of a subwoofer is dictated entirely by the enclosure and how it couples to the acoustics of what particular vehicle it is in. A sub system may sound great in one vehicle and just so so in another.
Now ,if you are having issues at a certain point with your subs, you need to determine what in fact the issues are.
What type of enclosure are you using Sealed, ported, bandpass, horn, etc?
What is the model, coil DCR and wiring configuration of the sub/s?
See, what I have found in dealing with people and sub systems (especially teens with their first system and for the record, I have a 19 year old son and 15 year old nephew as well as their multitude of friends who always end up at my house to work on and tune their particular set ups) can be summed up simply in a few different problems.
1:To little power or to few/small of a subwoofer.
They are trying to beat the laws of physics with what they are wanting. The person is working on a very limited budget and trying to get window cracking results with 500 watts and under or a single 8" sub in a tiny sealed enclosure
2: Wrong enclosure.
Sealed enclosure are the easiest and most forgiving of designs. There are trade offs. You want low end? Go ported. You will get louder and lower (there are trade offs as well but I am trying to go simple here) on less power in most circumstances.
3: Wrong settings on head unit/amp.
You have bass boost maxed, crossover set incorrectly etc... Usually trying again to get the most out of a limited system, as well as not understanding what a bass boost control really means. We have all been there
Can expand more but just wanted to touch on these three.
jkem, have you reset to zero any boost functions of the head unit and amp?
Set gain on amp at about half. See if distortion levels drop. Sounds to me like you are either clipping the head unit or overdriving the subs. Am guessing the amp though. This will affect a bit of the low end. Especially if you are running a sealed enclosure. If ported, check and verify that you have a SSF and that it is on. If you are unloading the subs below the tuning frequency of a ported box, you can eventually damage the subs and it will also not sound good.