i got a 8" powered bazooka bass tube in the rear, and they do offer a volume control that you can remote mount. the price would be approx 250-275 for the speaker and the control, more for the install if you dont do it yourself. it sounds good in the car, can hear it very little outside. it is very efficient, and small in size, so you still have most of your trunk to use. and because they use smaller drivers, they have better musical transients than if you try a 12 or 15
they call it either a "speaker level converter" or "low-level converter" that sends the audio input into the amp. no, it is not as clean, because you are taking the semi distorted analog signal from the stock deck and then working with that. if you are looking for an audiophile sound quality, you need a new deck with RCA outputs, but for what you are looking for is just recreational listening, it is much more cost efficient.