Ouch, that's a tight price range and a painful system you got there.
My advice would be to get a nice sub, probably the HSU STF-2, and then see if you can get a LCR, or even just the LR for now.
The STF-2 will drop you $350 + shipping. You will find the most improvement from buying a sub first. The only other sub I can think of off the top of my head that would be worth purchasing in that price range would be the BIC PL-200.
Or, you can move your budget up and spend the $500 on a great sub, and you won't ever have to think about upgrading your sub for 15 - 20 years.