You could try gently with a credit card wrapped in a tissue or similar, but the risk of scratching the veneer is real, and considering how great the condition is on these I think a less sharp, more purpose made implement would be better.

Cap replacement on a speaker this old is probably a good idea - the crossover is just first order I think so not at all intimidating to work on.

I am quite jealous TBH. Lovely speakers you have there.