Havent opened up the sub enclosure yet, but based off this old thread, http://community.klipsch.com/forums/t/2456.aspx?PageIndex=1 I'd have to guess that the drivers in the sub are rated at 4ohm each & wired in parallel (so a 2ohm load, hence the 1.8ohm testing)?
That'd cover building a new enclosure to suite new drivers...
However doesn't really help with replacement drivers in the standard enclosure... Without calculating the details of the enclosure (i.e. volume, port tuning etc) ya can't really do a swap with any ol' 4ohm driver & expect good results. Different speakers are designed for different enclosure volumes, sealed vs ported etc... So having actual specs of the standard speakers would be awesome...