You might try 100 or 200 ohms or so. Measure the discharge time. Don't fix amps using headphones on your head. RMS per channel on the bill. Customers love this information and it costs virtually nothing to give out.
Nobody wants electrical shocks either.
Don't be superstitious but a prayer can work wonders. If the amplifier has a speaker protection relay - you will want to short the contacts for testing purposes. You can't wait for the relay to normally turn on to see if the amp is working or not. If a fault still exists, by the time you get the variac turned up enough to turn the relay on - it may be too late. Solder 2 short wires across the contacts.