People do a line-level output converter and add an aftermarket amp with new speakers and a subwoofer. The stereo does not have a dedicated subwoofer output. Nor does it have any line-level outputs. Or, you can get an amp that accepts the high-level speaker outputs and not worry about a line-level output converter.
If you don't want to spend that much money, you can replace the speakers first and see how that sounds. I did that and the bass is much more solid and less boomy. If you have the 6-speed with the Premium Audio, your front speakers use a dedicated crossover on the back of the head unit. So you might want to consider component speakers up front since your front speakers are wired differently than the standard Accord.