Realistically probably not a whole lot more than rated.
A few words of warning:
Those old class A/B amps are not efficient in size or current demands. They'll run hot and draw a lot more than modern class D. Though on 500W range that's not such a big deal as far as taxing your electrical system but you'll need to mind is has some space around it for shedding heat. Also electrolytic capacitors can start to fail after about 20 years so if it's that old it could give up the ghost at any time just from old age of components on the board.
If I were buying low-ish power sub to get loud and sound good right now I'd look into the Adire re-boot lineup. The ones from 20 years ago are still in circulation and are competitive so I'd have high hopes for the new releases.
www.adireaudio.com