Hi There!
As jefe1 said I also have a Proton SD-1000 purchased largely on comments made by former member Disclord. I was lucky to get one as brand new old stock on eBay. In perfect condition though I did have to replace the remote batteries. On that last point I hope you have a functioning remote with it otherwise the unit is totally useless.
Ok, the downside. It does not decode SQ or QS or any other quad format properly. It was not designed for that purpose. This is a very sophisticated perhaps state of the art for the time analog Dolby Pro Logic decoder. DPL analog matrix decoders were terrible for playing back stereo music. Not just Aphex/Proton tried to improve this but others did as well. As a manufacturer complied with the specific rules of Dolby decoding they could then do pretty much what ever they like for stereo play back. The Proton SD-1000 owners manual says:
I have used test records for QS as well as my own test signals created in AA 3. Except for cenetr front or center back not one emanates exactly as it should from a QS decoder or even from a DPL II decoder adjusted for QS. SQ test records are the same. Listening to SQ in cinema does at least get the front channels correct.
The good news: It is an extremely low noise low distortion analog decoder & I have never heard a decoding artifact like pumping. Inspecting the interior it is one of the most complex & exquisitely produced PC boards I have seen for that time. It is only the 2nd decoder I know of with a separation control. It is so cool the unit knows what speaker config you are using based on what is plugged into it.
In regards to last I'll say this. After testing the unit & auditioning much in 4/5.1 I was kind of disappointed. It didn't seem to offer any more surround excitement than say a Fosgate Model 5, that I also have. On a whim I decided to hook up a center back speaker that I have never done before. The transformation was rewarding! The rear sound stage seemed more vibrant, the depth seemed improved. Remember it's just not about adding another speaker to center back but also what is extracted from the rears to create that center back. It really helps when the the three speaker feeds in front are complimented by the three speaker feeds in back. I think the Proton SD-1000 will be supplanted by the new Surround Master for me. But if you can use use it for stereo decoding in the six channel mode I think you might really like it.
Side note: the sub woof output has too high an xover point and I estimate only a 6dB/octave roll off. It makes everything tubby boomy and awful. Just leave it unplugged.