ok so you can cram some more capacitance in, but has the owner reported an actual problem ?
Why does it matter if there is currently a "problem" or not? This wasn't a
repair question. The owner wants to change the capacitors. Maybe he's going to install it into a permanent installation and doesn't want to have to worry about component life for another 20 years. Who cares
why? That is a completely different conversation.
once upon a time diodes would be supplied with a maximum capacitance rating they could work wih for this very reason,
Most certainly! Especially tube rectifiers with capacitor input rather than choke input filters, you most definitely don't want more than the recommended capacitance on the rectifier (typically about 20 uF for a small receiving-tube sized rectifier) or you'd blow the daylights out of it in short order.
This amplifier presumably has one or more beefy bridge rectifiers which can easily handle more surge than they were previously handling and it even has soft-start. He's not going to be blowing up rectifier diodes adding some more filter capacitance in this case.
hence your 20 ohm series resistors, this kit was well thought out, but apparently the owner knows more than the designer. Just make sure you apply gold coloured sticky sheet to the outside and charge him 100x the actual price of the caps and you will be ok oh and you will have to come up with a bit wank word for the new technology you have applied, do that and he will be happy
Maybe the owner is after audiophoolery and maybe not. The original question, though, wasn't whether there was some audible benefit from increasing the capacitance and we have no idea why the owner wants to do it, it was whether it would work or not.
Replacing 20 year old electrolytics isn't some kind of insane, crazy idea, especially if they have been in constant service! That is reasonable maintenance philosophy, IMHO, and going from 10,000 to 15,000 uF is also reasonable from an engineering perspective.