The Servodrive Subwoofers have been in service scince the Late 80's, There are many pluses and minuses to them. On of which is cost. Another is they have higher distortion at very low volume levels due to moter and motion converter stiction and jerkyness.
Check this site out.
www.servodrive.com They still manufacture the BT7 and the Contrabass.
There have been a couple of what I'd call failures in the servo driven speakers like the Acoustic SB 2.6, and the cyclone subwoofer.
I have no idea what year the recording bass is but I think it may be pre triumph bass. So maybe 68-71?
Anyhow thanks again
Antone-