Should I go for sub-woofer or new speakers?

I would also recommend the REL Strata III, an excellent sub. I doubt you could equal its performance by adding $800 to whatever you could get for your existing speakers. Just be sure you are willing to spend the time and effort to get it set up properly. It's not rocket science but does take some time.

I sometimes use my sub with my small apogee ribbons and it does give them that full range sound and is very nice change of pace with them. As Thsalmon says,works best if sub is placed between the speakers. In my case,between,to the right and a little bit back of the mains. Hometheater placement does not seem to matter as much which is where my sub is used the most. The REL or M&K's might just do it for you,they are both very good and fast subs.

I thin u answered your own question...you like what own...and want more bass...wah-la...subwoofer...and for $500...the HSU vt-2 would be the one to checkout...I also hear Aidre has a good one also...at $400...you could spend more...but these are half the price of anything approaching musical sub sound...

Thanks so much for the feedback.

I have to disagree with all of you (I love that :)). Improving your speakers will not only improve the low-end, but also mid and high frequencies, while adding a sub might improve on your low-end, but will also be very unpredictable. The golde rule is: listen to the two set-ups at an good dealer, then arrange a try-out period for your own listeningroom, and then buy with confidence.