There are detail differences among the various versions, chiefly the cabinet and grille styling and the internal structure of the cabinet. Most have a wood or wood-looking cabinet. A few were built having a glossy “piano black” finish.
- Series I—-1968 to 1973, acoustic suspension (sealed cabinet), drivers use 4 fasteners each to hold them in place.
- Series II—1973 to 1976, acoustic suspension, improved high frequency response. Drivers use 4 fasteners.
- Series III–1976 to 1978, Acoustic Matrix (ported cabinet–takes less amplifier power to drive to the same volume level) Drivers use 3 fasteners.
- Series IV–1978 to 1983, Acoustic Matrix, improved high frequency response. Drivers use 3 fasteners.
- Series V—1983 to 1987, Acoustic Matrix, drivers use 3 fasteners.
- Series VI–1988 to present, Acoustic Matrix, drivers use 3 fasteners. This is current production, as of April 2013.
The specification on these has always been awesome, however early models have driver edges made from foam which can crumble away to nothing, so any early sets should be checked thoroughly. We have re-edged a few sets with varying results, later sets do not suffer from this. You will need a really strong amplifier to get the best from these 300w per channel minimum. You will need a controller with these.