The separate bass/ treble control units are the earlier ones with metal grills and 5 inch long throw woofers, the single tone control units have cloth covered plastic grills and a 4 inch long throw woofer.
Surprisingly my original units from Nov 10, 1994 haven't needed any repairs at all, ( the foam speaker surrounds are perfectly fine ) but the later 4 inch units I just picked up have no foam left ( I believe the clear plastic body makes these display or demo units for use in stores, but I might be wrong )
I've replaced one speaker with a shielded, non-long throw 4 inch woofer from Goldwood Sound (GW-204/4S-2) just to get it working ( I'm using one as a portable PA system for a part live / part online Zoom meeting ) but the output was no longer balanced ( tweeter too shrill )
So I added a layer of foam rubber over the tweeter to tame the output a bit. Sounds much more balanced, ok fix for a PA but not for music.
I'll see if I can re-foam the original 4 inch woofers and compare. These are similar to the units on the back cover of the catalog referenced in earlier posts.
Update: Purchased foam surrounds and adhesive on Ebay and followed the seller's printed instructions. These were the first speakers I've ever re-foamed and the "operation" was a success. The sound was much improved vs the Goldwood woofer. The bass isn't as deep as my 5 inch woofer model, but it's a nicely balanced sound worth the time and money I spent on them ( yeah, I love to tinker ).
I'm powering them off of a 15 amp/hour 12 volt battery normally used for emergency lighting and a DC Barrel type power plug with a 5.5mm OD and a 2.1mm ID.
With a rechargeable Monoprice Bluetooth Long Range Transmitter and Receiver IIIP I now have a portable stereo setup for PA or an outdoor movie.
