How did you connect your Cervin sub? With a Pac/LC2i LOC? I might be interested in a under the seat setup while keeping the factory HU.I got the Cerwin Vega 10 inch under seat active powered subwoofer. It fits under the seat, the mat that goes on the back floor does not fit to well anymore. The seat will probably have to be removed to anchor the subwoofer but the seat did not have to be lifted at all. What is nice is you do not have to give up cargo space and no one can tell it is there. I had it professionally installed and they used double sided tape to stick the wired remote on the side of the center console so I can easily adjust the bass on the fly as needed. There is a blue led light on the remote knob but it is a fairly soft light.
Here is the link https://www.amazon.com/CERWIN-VEGA-MOBILE-Powered-Subwoofer-Enclosure/dp/B01M5LBQTS/ref=sr_1_5?keywords=cerwin+vega&qid=1562736032&s=gateway&sr=8-5
I bought it from Amazon awhile back and I have had no issues with it and it has really enhanced the sound of my car. I have been debating whether or not to install a second one under the passenger seat but that might be overkill.
From the few youtube videos ive seen, can use a Line Out Converter via High Level Input(usually rear speakers) to RCA. I guess I might try on my daily and see what happens.I had the subwoofer professionally installed but I did not use any PAC/LC2i, I believe they just spliced into the wiring. To me this was the cheapest way to improve my sound without tearing out the HU. But I also added aftermarket tweeters to the front doors so that also enhanced the sound. This car just came with 4 door speakers so the sound was real muddy and no real clarity on the high or low end. By adding the tweeters and the subwoofer really gave some oomph to the sound coming out of the HU. I mostly use my iPhone to listen to either iTunes or Amazon music rather than the radio.