Should I go for sub-woofer or new speakers?

I now have Linn Ikemi cd player, Naim Nait 3 amp, Royd Envoy bookshelf speakers. I like the Royds but want more roundness/bass presence. Also, could anyone recommend a decent pc for this system. Thanks
I think you have to demo to find out which you prefer. I added a Rel Strata to my system and it's the best upgrade I ever made. Music has much more impact, venues have real scale. However, even used it was $800 which is a lot of money .. if you add the $800 to what you'd get for the Royds you'd get a significantly more capable main speaker.

So as I said at the start I think either approach is valid, but you really do need to try a sub before buying. Also my main speakers are floorstanders, and I think a sub might be a little more tricky to integrate seamlessly with bookshelf speakers, due to a higher crossover point, and the possibility that this might make the sub a bit too directional.

Sorry I can't give a definitive answer, but I really think it's a matter of taste.
I'll second the REL subwoofer recommendation. Their subs do well with music and seem to easily integrate into a system. I'm not familiar with the Royd speakers, but when I have used a sub with bookshelf speakers, better results were usually obtained by placing the sub between the speakers, setting the sub's crossover point as low as is feasible and not cranking the sub level up too high.

As for power cords, note that the Ikemi has a switching mode power supply. So-equipped components don't seem to be greatly changed by power supply tweaks in my experience. I tried a few power cords when I had an Ikemi. Most seemed to make no difference in the sound. I thought that there were perhaps some very subtle improvements with the modestly priced Audusa Eupen cables however. Sorry, but I don't have any experience with cables and Naim gear.
There are a lot of good subs, and a lot of good reasons to use a sub/monitor combo. I use an M&K MX700 which, like most M&K subs, is controllable and can "get out of the way" of your beloved monitors. Borrow a sub if you have to, but you should really give one a try.

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 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 $ 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.
Where it my system, I would just buy better mains, too. When you factor in ALL of the cost associated with adding a sub, which is much more then just buying a big black box, you could pick up a really nice set of main speakers that will likely eclipse the current setup w/woofer, likely done for less $$$ to boot.