Advice for next upgrade?

Hey guys, Just a newbie here in terms of audiophile equipment. Lately i've really started to appreciate nice audio quality. I've bought myself some klipsch towers that sound just amazing with an old sansui stereo receiver. Those are in the living room though.

The real magic however, happens in my pc room where i listen to all my music. Keep in mind, im a student who works part time, i cannot afford tube amps that cost $500 for 3 watts a channel.

I have an insignia receiver, and for what it's worth, it sounds damn good. Im sure there are much better options but again, i have a little budget.

Often i go on craigslist and look for nice equipment and found myself in amazing luck when i picked up a couple of polk bookshelf speakers from the 90's or 80's. Dont know what their name is Here is a pic: http://i.imgur.com/ZIHHc.jpg

I also have a polk audio PSW505 sub that sounds (IMO) very good, but a little boomy. I also will get a DAC soon.

So what, at this point, in an affordable range, could i upgrade to make sure my audio is even crisper. Thanks guys!