i ended up not setting my system up where there was carpet, so i never added the spikes, but i was planning on just using the existing screw-holes on the bottom of my sub, instead of worrying about t-nuts. iirc, the threads of the screws holding the feet on my rsw15 are "1/4-20", but they may be different on the ksws, if those even have screws on the bottom. i emailed klipsch for this info, btw, and they answered quickly. so fire off an email and ask 'em.
here's a link to some spikes with different thread sizes, btw
http://www.oregondv.com/15spike.htm