UTV Stereo 10” sub kit. I did this in my 3 seat Ultimate, and it sounds amazing. The sub takes care of the low end fill, you can turn down the bass on the EQ to make the highs and mids sound good with no distortion. Money well spent, sounds slightly better than my Xpedition northstar with the JBL system it had, and that sounded great once you got it dialed in.
Only issue is, with the thumper fab nerfs and the bracket for the sub, the passenger door doesn’t close as nicely as the driver side. The extra thickness of the lower part of the door sill is the issue.