That is such a broad question.... First, i've seen subs cost anywhere between 10 and 1000 bucks. Second, i've still got my factory speakers in my car even though i have a set of components i pulled out my last car, i just have been too lazy to put them in. But what makes a world of difference is running them on an amp. with proper crossovers in place, you can run your factory speakers just fine for a long time (as long as you don't get carried away with the volume knob of course). Mine sounds good enough i don't even care enough to pull them out; but mind you, my factory speakers have components in the front doors. It really depends on many different factors. As far as recommending anything; only buy what you want to spend. Like most electronics, you get what you pay for. Some of the higher end items could really sound like items far less expensive though. It really takes a bit of messing around with different set-ups before you discover what you like.
