I've not done a Subaru but I built sound quality cars and competed in sound quality competitions for years. And there is no way to say this w/o sounding braggish but I've taken numerous first place titles in both national and world finals. Maybe I can make up for my stupidity in other parts of the forum in this section : )
The biggest influencing factor would be your budget and whether or not you can do the work.
But if you have a stock head unit and just want to upgrade your speakers w/o re-wiring your only option is the set of upgraded Subaru speakers which can be found periodically in the classifieds here or on eBay. You would just want to make sure ahead of time that those aren't what you already have installed.
You also could possibly add (depending the year of your car and head unut) the Subaru subwoofer that mounts under the seat. I installed one just to see how it performs and for the price and ease of installation it's great.