Linda,
As I understand it, the two versions are very different. Each is actually tuned to the operating frequencies of the particular line of machines. The XLT, XL Pro, 6000, 5900, etc... use a 6.59khz coil and the DFX and MXT use 3khz or 15khz coils (multiple frequencies).
I suspect using the "wrong" coil on any of the machines will result in severe loss of performance and depth of detection as the whole system will be working out of tune (like trying to pick up a radio station in your car which not tuned in properly for example).
This is also the reason that coils do not usually work between brands. I do not know if the use of a coil not tuned for a detector can cause damage, but I do know that if you use a radio transmitter with an antenna far enough out tune, the reflected RF power can damage the transmitter. And that is the same principal used by our detectors to "transmit" the signal into the ground to detect metal so damage may be possible (not sure however).
