I replaced all the electro caps less than two years ago. Amp
worked fine up until now. Polarity was and is correct. After
cap blew today, new replacement cap over heated. I shut off
power before it blew out.

It's remotely possible the replacement was defective. If the voltage across
the cap's location is correct in level and polarity, then that can't be the
problem.

Question... How safe is it -- other than the cap blowing -- to operate the
amp? I think you need to put a 'scope on a number of points and see what's
happening.

I don't mean to hassle you on the question of polarity, but... Japanese caps
generally have a black line on one side pointing toward the negative lead.