I had a similar problem in my phono stage. It would light up on the low power start up, then blow the fuse when the high power switch was flipped. One of the diodes in that circuit measurted fine by itself, but showed a short under load. I suggest replacing all 4 power supply diodes as a start, if you can find out the part numbers.
Of course, it could alwyas be something else, like the transformer, but I suggest starting small and easy first.