I am busy refurbing an old valve radio for a mates dad.

It works OK, but hums a bit.(the radio, not his dad)

I have some new electrolytics for the power supply, and am wondering
whether to remove the old mans capacitors and put in new ones, or simply
wire the modern replacements across the old one.

In short, so old electrolytics simply lose capacitance, in which case
adding some more can only be good, or do they also develop extra
leakage, in which case removing the old (electrically) is to be preferred?