For mastering applications, M/S can be a tricky circuit to implement if you want to keep your equipment on the same settings for stereo or M/S processing.
When converting to M/S you will get 6dB of gain in the M channel. This has the potential to distort equipment if it is being driven hard. So to compensate for this, many mastering M/S processors drop the signal by 6dB when encoding, then you have your ananlogue inserts, then it will boost the signal by 6dB on decode. Some resistors will do the trick. I'd personally use OP-AMPS over transformers as they are more predictable in their behaviour, and have greater common mode rejection.
Then you would also want to monitor the Mid or Side signal pre/post processing.
So the circuit will quickly go from this:
Pico Compressor Audio Design Forum • View topic - Mid Side M-S Matrix Uses No Precision Resistors
To this:
Pico Compressor Audio Design Forum • View topic - Mid Side M-S Matrix Uses No Precision Resistors
On top of this, an M/S 'spreader' circuit. This differs from just boosting or cutting the side level for more 'width', as this method affects the mono signal as well.