Go to Radio Shack and buy a Basic Electronics book. (Seriously)
Hehe. I'm mostly here for the popcorn, but I'll drop a few useless comments.
EVERY bass player who is serious about playing (like, serious enough to buy expensive gear, like, say an SVT or an 8x10, etc) should have some basic electronics knowledge. This is more for THEIR safety than their gear.
In medical electronic engineering (at least here in AUS), you're required to use isolation transformers in any equipment which touches a human. This is a good rule, because even though one can look at the circuit, run the sums, and work out there is extremely little probability that any unexpected (well, expected unexpected, I suppose!) behavior would lead to a human being endangered, it's still just better to be on the safe side when it's people we're dealing with.
This leads me to two points:
1) how much do you love your vintage gear? They're not making more of these things! I'd treat mine like a human - not worth risking it, even if it
seems safe.
2) A bass is technically an electronic device which touches a human. There's always the possibility with high power electronics (particularly if they're damaged!) that something goes wrong in such a way as to endanger YOU. That's right, amps are friggin' scary. You wouldn't stick a fork in a wall socket, right? Well - something akin to this
COULD happen with your amp, if it fails in just the right way. Now, as was REALLY helpfully explained by
@aborgman (thanks mate!) the likelyhood of this type of failure is very small - there are several more likely failures which
SHOULD happen first. One might compare it to being eaten by a shark at the beach - possible, but not likely enough that you won't swim. If you see a shark, though, you might change your mind, cause your odds just got worse. In your case, you haven't
seen the shark, but someone else reckons maybe he saw one yesterday.
When I was doing repairs (in a previous life in Australia), the electrical licensing authority required me to have a megger.
^^^^ This is why 'straya is a bloody rippa country
