It appears that the tube must have "run away," that is, it ran at excessive current, which raised the bias to a voltage higher than the bypass cap could handle - thus shorting the cap.
The question is, why did the tube run away. You said it has a scratchy sound, but it was not clear whether any actual music came from the tube. That would be a clue - if there is no music, then it is likely there is a grid-to-cathode short inside the tube. That's fairly rare, and is easily solved with a new tube. However, if the grid bias came disconnected (usually that would be a broken 220 ohm grid stopper, but could be a bad connection ion the grid to negative bias resistor) you might get runaway with music still sort of coming through. If the tube had lost its vacuum, then grid emission could do this for a short while, but you would see the silvery getter deposit turned white.
To answer the question, yes a 63-v capacitor would probably survive better than the stock 35v. I say "probably" because electrolytics run well below their rated voltage will often develop a memory effect, and lose the ability to take their full rated voltage. Conventional opinion is that 50% to 80% is ideal, so 20v bias on a 63v cap (30%) could see this effect.
As I said, an internal short is rare - this is the first time I've heard of it in the 8 or 9 years the SEX has been in production.
