There are a number of good mods you can do to this amp, many posted on the internet, however I discovered that there is a German self build company offering a kit called The "Jim", https://www.tube-town.net/cms/?DIY/LoW-Projekte/Jim
it contains all the best mods to the amp from the internet, many of which I had done individually to my amp. I enclose the circuit diagram of the German amp as it's really the Class 5 with all the best mods, just review the Marshall Class 5 Circuit to the Jim and then implement the mods that you think you will like, or just do the lot, makes the Class 5 a great little amp when completed. Not sure how they get away with selling this amp as it's identical to a Marshall Class 5 with a few mods here and there. I'm not going to list what all the mods do, there is so much on the net regarding this, all I will say is that if you change your Class 5 to match this circuit you will not regret it one little bit.
The last mod I did was the Master Volume, as the attached diagram, you have to cut the PCB track and use screened cable to the pot wherever you decide to mount it, I put mine on the back panel as I did not want to drill the face-plate. One thing that this mod did which surprised me was to seriously reduce the hum the amp had, it wasn't bad originally, but not good either, inserting the Master Volume mod now makes it very hard to tell when the amp is switched on, great job. Whilst you will never get JCM800 sounds from this amp, it's not designed to be high gain, you can get some nice low volume warm tones from the amp. With all the mods I did, I really like the amp now, it what I always hoped the amp would be, works well with Single Coil or Humbuckers now, before I only liked it with single coil, too muddy and dark for my taste originally. Whilst as always sound is a personal thing, all the mods do make the amp much more flexible than the original Marshall design (sorry Steve - no personal criticism intended).