My older Soundstream XXX has a 4" coil. The 18's, I'm not positive.
After I reconed mine, I dropped it in my box, hooked it up to my SAZ-3000D, and fried one of the coils. When I dropped the sub in, the sub got hemmed up on the cutout for the enclosure and one of the wires came out from the basket terminals.
The one coil lasted a good ten minutes or so before it finally gave. 8500RMS, I'm not sure 100% on, but it can take a bit of power.