It will be tough (impossible?) to find a sub for $500 that will do your system any favors. Stay clear of the typical HT subs in that price range, they are boomy and don't integrate well for music. My advice is hold off for now and save your pennies to get a used REL Storm III (~$1000) when your budget allows. Run it off of your amp's speaker outputs to the sub with the supplied REL hi-level cable. You will be much happier with the results.
In the mean time, try placing your speakers closer to the corners or walls and see if that helps fill in the lean-ness.
Good luck.
In the mean time, try placing your speakers closer to the corners or walls and see if that helps fill in the lean-ness.
Good luck.