the slime in capacitors is installed at the factory. They are sealed in the aluminum can by rubber, except on special high cost epoxy sealed capacitors. While some high end medical and boutique hifi equipment had long life capacitors, nobody on organforum seems to have found any. If you like vintage rubber, try driving around on 30 year old tires. You measured the air pressure before going out, so that means the tires are good, right? The heat of operation causes vapor pressure in the water contained in an electrolytic capacitors, boils it out past the cracked seal, and eventually they short and pop open.
Shorted capacitors burn rectifers, voltage regulators, or transformers. E core transformers over 20 watts start at about $75 now and go up in price fast.
I don't test this stuff, I throw it away and buy new. I test them with a calander. >20 years = wrong. I bought some 3000 hour 4700 uf 100 V capacitors for $5.50 each recently, so they are not all that expensive. 10000 uf up is expensive, so I assembled 4700's into subassemblies for my 10000 uf caps, saving $15 each. Don't buy any 1000 hour or less caps unless you plan to sell this amplifier soon. I've had to replace the caps in my power amp 3 times in 40 years, although until the internet nobody stocked long life caps much.
If you can afford to throw your amps away after you blow slime all over them, have fun with the old caps. Alternatively, you can disassemble them after the explosion, clean the slime out, and reassemble with new parts replacing any you burned up.