Man oh man, I just found a pair today.
This pair has cleaned up rather nicely. They have a lifetime warranty, so you can find 'em at RS's web site. The stated FR is 20-20K Hz. 8 ohm impedance.
Speakers include a 12" acoustic suspension 'heavy magnet woofer with brass voice coil,' 1 extended range high compliance tweeter & two midrange speakers.
The enclosure is a sealed oiled walnut & non-resonant chip board, internally braced. Removeable grill (mine uses screws).
Midrange and tweeter controls on the back. They weigh 38 lbs.
They were discontinued in 1977.
I'm just gonna get up and switch the tone controls on the SX-5590 on and listen for anomalies.