Quadraphonic records (LPs to you and me)

The demodulators are for the CD-4 LPs.

Ok but a proper decoder is what ...?

@tatyana69 see the link below:

https://www.ebay.com/itm/Vintage-Sony-SQD-2070-SQ-Decoder-Tested-Works-Great/292327234714?hash=item4...

So the quadraphonic lps need a special cartridge ... and phonostage then. So presumably playing with a normal cartridge and phonostage some channels are missing, hence maybe a weaker sound for a number of reasons, presumably, from what you are saying?
Must be for a very limited market place these records then

I feel I am either being stupid or missing something here
If the Bryston decodes elements into separate aspects to surround speakers I was wondering if the input already into 4 may provide an easier or more varied decoding

You are missing something- the history.
4-channel was a thing that showed up in the 1970s. It was very much a precursor to surround sound and home theatre.

There were several formats- several forms of 4 channel tape, 4 channel encoding on FM stereo (usually SQ) and three competing formats on LPs.

The three LP formats were CD-4, SQ and QS. All are compatible with 2 channel playback. A decoder was required to extract the 4 channel information. In the case of CD-4, the rear channels were encoded on an FM carrier track that was outside of the audio band (and so needed a special cartridge, although many modern cartridges have the bandwidth and stylus that can do the job). The special cartridge saw the introduction of the Shibata stylus.

QS was the Sansui encoding, and SQ (if memory serves) was Columbia. They used phase information encoded in the grooves to extract the rear channel information.  Because of the method of phase encoding, its not surprising to hear that they don't sound as 'strong'.

All of these systems are analog based. If I understand correctly, the Bryston is not- so it can't decode these LPs. You'll have to look out for the decoders on ebay- they are easy to find and not expensive.