Thanks Kurtis!
I had an X9 not working properly and based on the symptoms and your advice scattered here and elsewhere over the years narrowed it down to an RF level and/or Focus servo issue. Not wanting to just "fiddle with the pots" over the last few months I've accumulated an oscilloscope, signal generator and even sat down with my phone and google translate app to translate a couple of bits of the service manual!!
Anyway finally found the time to work on it over Christmas/NY break and happy to say now have it working
Had to adjust the RF level as well as both the Focus and Tracking gains (which I also assume work as per the RF level adjustment) so seems certain to be a case of the laser weakening over time. All the other adjustments I could measure/check without the muse test discs seemed to be in the ballpark.