I would imagine the 8C's would be very difficult to integrate with a sub since they go down so low. Unless you have a very large room and roll off the bass in the 8Cs, what your libel to end up with could be very muddy bass.
The best way to integrate subs with (any) speakers is to highpass the speakers. This should work equally well with the 8C as with any other speaker. If one does not have that capability, I agree it would probably not be worthwhile.