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.
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.