I used to sell Infinity speakers in the late 1970s and I remember that those Quantum Line Source speakers had a massive woofer with a dual voice coil. Replacing one of those could be difficult. Of course, if it only needs surround replacement, you could get a very good deal. As I recall, there wasn't enough power on earth to push them to their limits. Very inefficient and a hellish load.
It was our Thelma Houston speaker.