Of course I replaced capacitors with ones that have the exact same capacity and voltage rating.
As for the polarity, I made sure to put them in correctly (it is labelled on the board).
I did the measurements again without any load, AC input is 48V with DC output 64V, and ripple is 12V.
With speakers plugged in, input signal and volume at max, the measurements are exactly the same and ripple is around 10V
I don't know how the measurements can be so different from the previous time but it is probably my fault.
Sorry about that...