The caps reside in each corner of the amplifier. The original caps are large and expensive, but a good option is to replace them with a matrix of smaller caps to the original value.
A matrix of smaller caps provides similar capacity, has a higher thermal and voltage rating and their response time is faster , your amp will sound better as well.
If your handy with a screw driver and a soldering iron you could do it yourself.
I owned a 331 years ago , this mod was done before I bought it.