I replaced just my headunit for a more modern one that has Bluetooth and HD Radio. Pioneer DEH-X7800BHS. It's not all a win/win, as these new headunits really need a constant +12V supply to keep the setup and audio settings which I'm working on next.
Sound quality is satisfying at this point with the stock speakers, because it's easier to access high quality sources (from the connectivity advantages listed above, as I don't use CD's anymore) and I spent more time tailoring the sound profile (add loudness and a bit more bass).
On the sub, you may also want to make sure the crossover is almost all the way so that it's not artificially filtering out bass inputs.