Should I go for sub-woofer or new speakers?

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.

Charlie