Hope someone can help.
I am unable to tell if the subwoofer in the RR is working properly.
When adjusting the sub to full up or full down I can't hear any difference.
The dealer had a "look" and compared it to another RR, and said it was working as intended.
Is as intended so that the driver can't tell the difference?

Not entirely sure what the audio system was b ased on in the earlier cars. The HK system in my 2007 is definitely noticeable when the subwoofer is tuned off.

Easiest way to prove for sure is to put an analogue meter across the terminals of the sub (using scale of 2-20 volts) and run the system. Note that digital meters won't react quickly enough. Oscilloscopes are best if you have access to one.

What I would say though is that if it's anything like the subwoofer unit in a D3 then no, you can't hear it working. What I did in my D3 was take the wires from the sub and put them into the inputs of a mono amp (high level inputs) and used the amp to power a box in the boot which had a 12" Genesis sub in it. Removeable when you need the boot space, decent tunes the rest of the time. Success is the ability to go from one failure to another with no loss of enthusiasm.
Winston Churchill.

Try finding an testing audio track that outputs just the low frequency range <100Hz (or even make one using audio software) burn it to disk and try it .... You may not be able to hear LF but if you put a piece of paper near the speaker you'll certainly see the physical effects of LF.

Hope that helps