IMO, reduce the capacitor value will has a bigger impact in sound quality, those 220uF capacitors are mounted into a 35mm clamp with point to point soldering. You need to find a 50uF cap with similar quality and diameter to fit into those clamps or you need to find a way to mount them into the chassis if the caps has different dimension. And the modification cannot be easily reverted!
OTH, install two UF4007 on the rectifier tube socket is a very simple job ( it takes less than 10 minutes ) and those diodes only cost a few dollars. Its also very easy to reverted.
Those diodes are functioned as a protection device to reduce the chances of rectifier tube arcing and also can prolong the life of those rectifier tube. BTW, the effect in sound is negligible.
My suggestion is go ahead install the diodes first, put in some new rectifier tube and see if it solve the arcing problem. If you don’t like the result, then go for the capacitor mods or install soft start device.
BTW, always wait as least 5 minutes to let the tubes cool down before you switch the amplifiers on again as @dekay suggested.