[GrimJosef]

I'd support Glyn's comments. Non-reversible changes certainly do devalue these amps and I wouldn't recommend drilling/cutting the chassis for any new connectors. It's perfectly possible, though, to disconnect the Jones socket, carefully insulate the ends of the wires (HT, heater) that you won't need and then fit a phono socket onto a small plate that will fasten in place where the Jones socket used to be. As long as you don't lose the Jones socket you'll either be able to sell it on as modified or reverse the change and sell it in original condition if/when you decide to.

Cheers,

GJ