Fortunately the tube is an easy fix if that's what the problem turns out to be. Here in the US, a tube replacement can run well under $1,000. That's certainly a pretty penny, but for an historic television system it may be worthwhile. And yes, the tubes are sourced from Phillips, but they are Bang & Olufsen spare parts and would have to be acquired from Bang & Olufsen.
There is scarcely anything in this world that some man cannot make a little worse and sell a little more cheaply. The person who buys on price alone is this man's lawful prey. - John Ruskin