To be honest, I've never tried that either, but with a USB stick it works like a charm ...

No, he didn't. External....

The Marantz CD6007 CD player does not support Bluetooth connection. It has a USB-A port on the front panel for playing audio files from a USB stick drive or mass storage device (MSC), but it does not support direct connection to a smartphone via Bluetooth.

As the SR 6011 receiver has both an analogue and a digital CD input, I see no problem in operating the two devices together

I don't have a Revox amplifier but a Grundig amplifier that is a little over 20 years old, but it works without any problems (simple connectivity via two cinch plugs). I don't use the device bus, because I don't use the remote control anyway... I therefore see no reason why the CD player should not work with the Revox amplifier.

I don't know, I only listen to CDs and he does that very well.