If you want a low key sub, just buy a 200 watt or so amp with an 8 inch sub. IF you have Pioneer, they sell a 10 inch sub at wal-mart for like 50 bucks, and the head units are only 150 bucks.
I wanted low key but ended up with kicker speakers for my mids with an older (when they were good) rockford fosgate amp pushing those, then two 12" PPI inverted in a 3 1/2' bandpass box. Had to take it out because there was no room to put anything. I now have an 8 inch Kicker that is plenty of bass...actually if you do want to crank it, it can be heard.
With an SUV like an X, it is basically like a rolling sound box. Get a ported or bandpass box so that the sounds can echo through your entire truck.