Actually, if you want "harder" then the goal is to be louder too. In that instance SQ does come into the equation. You don't want a sloppy sound to ruin the the thump. This is how it works...
Good Recording + Clean HU /Good SQ Output + Low Distortion on Amp + Well Made Speaker Designed for volume = Hard hitting bass.
I'm certain you have most everything under control up to the speaker aspect. You definitely have power and I'm sure the signals are clean enough for your goals. So either the Kicker speakers(I personally don't like them, sorry) aren't installed properly with poor mounting, bad box design, and bad location or they just aren't up to your task.
Before deciding to spend money I think you should look into poor installation first. First, go through and figure out if you're able to provide enough power to the current setup. Then use those links I provided earlier to see if your power runs are long enough for the output necessary. After that, tackle the enclosure. Go to Kicker and see if you can figure out optimum air space for your speaker. Then measure out that box to see if the volume is incorrect. If that's good then be sure to look at the box and if it's sealed properly. Last, try moving your box around the vehicle(if possible) and listen to it. If your box is ported then what frequency is it tuned for? In addition, try low passing your sub to 120hz and see if that makes you happy? Don't play with gains if you don't understand that yet.
I'm working on my current build but still have stock speakers. I've only installed a new HU and people think I've added a subwoofer. All the speakers are still stock too. I just "tuned" a little and the sound is a world of difference. A good set of component speakers can have a high pass of 60hz or so. 250hz to ~60hz is considered "mid-bass", from a 6.5" speaker in my case. Typical 3-way systems have the tweet running from around 20,000hz to ~3001hz. The mid range is ~3000-250, mid-bass ~249-60 and subwoofer gets ~59-20hz. Parts are 10% and install is 90%.
You don't have to spend more money on parts just yet. Play with what you have to see if you can make it work. Deadening will also help in this endeavor.