I know I'd be much better off upgrading the stock head unit but I really want to keep the interior stock looking. Given that, what are my options? Are there line outs from the stock head unit that I can connect to an aftermarket amp? I'd like some bass, the stock system has 0... and I wouldn't mind replacing the rest of the speakers as well (but of course keeping the factory speaker grills).

 

You can get a double or single DIN JDM OEM replacement piece that will enable you to go aftermarket while still retaining a mostly stock look. IMO the stock deck looks pretty cheap and going to an aftermarket 7" Screen NAV unit with the JDM double DIN kit looks much better and more befitting a +$30K car.