Amplifier trauma

Having got my little 70s beauty home from the post office pickup point I plugged it in, and found that it only gave a very small fuzzy amplification, regardless of how loud I turned it up.... so I emailed the vendor in disgust (and luckily was very polite). They emailed me back a couple of hours later to say "did you find an RCA cable in the box? You need to run that from the pre-amp out to the main amp in." Guess who's never owned that Hi a HiFi before? Of course I've seen the little metal hoops that are lodged in the back of some other people's amps, but never thought to ask what they were for...

Anyway.... I played all my new German records to myself VERY LOUD, and then the next day plugged in the computer (via a very long RCA cable), and shortly after plugging it in (after hearing the usual crackle that comes from pluggin an RCA into a mini-headphone jack), the bloody thing died..... $100 and 5 days later, at a very rustic hole-in-the-wall repair shop, I now have my amp back working - and it sounds GREAT.... but I'm not game to plug in the computer again. (And I'm reflecting that this is what might have caused the demise of my previous amp.)