Have they ever been serviced? If not they will be in desperate need of a recap, which may sort everything out. I recapped some M3's for a friend last year. Unscrew the plate amps from the cabinet, disconnect the umbilical. Pretty easy to do if you can wield a soldering iron.
God photos here: https://www.meridian-audio.info/viewphoto.php?compid=32
Out of interest what is the white stuff and why is it a concern? Could I just clean it off or does it hint of something more progressive?I see some worrying furry corrosion on some of the leads.
I would start by replacing all of those small black colour electrolytics and a few resistors around that Red "KS" label and the orange and yellow wires where the board is charred and the resistors look cracked.
Looks like salt corrosion. Do you live near the Sea? It's not good, all needs cleaning with isopropyl alcohol and an old sonicare toothbrush head.Out of interest what is the white stuff and why is it a concern? Could I just clean it off or does it hint of something more progressive?