What Will he be doing that the other gent did not ? You may consider if possible the new Sabre 32 bit chip. Pulse transformer [75 ohm] at digital input and removal of any digital filters. Remove all circuitry for Video if present, a Paul Hynes power supply and voltage regulators.
A separate power supply for each stage of regulation would be ideal. It depends how far you want to take it.
If it were me I would use the Marantz as a transport and look into a Bolder modded Art DIO. Beat the new Oppo SE hands down. Or a Oritek DAC. Just playing the Devils advocate here. As you have already experienced one does NOT know what the sound will be until after the money is spent. It can be neverending.
But but the price is attractive so what to do ? Tubes, well then maybe.
charles