I have had an issue with the volume of my subs, for which I opened another thread (Little volume from rear speakers), but for which I think it might help some of us here. Because it has something to do with the lack of subs volume/power.

Because our radio is not really an intuitive type, I kept pressing and holding buttons to find out what happens.

You can select sound settings by pressing and holding the OPT button for 3 seconds, until USER appears on the display. By turning the right rotary/push button you can now select USER 1, USER 2, or USER 3. The specific user memory saves the sound settings (BASS, TREB and BAL). Nice, nothing out of the ordinary here.

But now comes the tricky part.
When you press and hold the AUD button for 4 seconds, the tone values for the set user memory (USER 1, 2 or 3) are set to 0 (mean value), LINEAR. Meaning that the factory compensation, specific for our XF, is lost and the radio is set to its standard values, LINEAR, being (whatever) all corrections set to normal. Frequency and volume corrections are lost. Not being BASS, TREB and BAL.
And the tricky part is that this cannot be reversed (turned back) on the radio.

I noticed the clear difference instantly by switching between user memories (USER 1, 2 & 3) and experienced it as a lost volume/power in the subs.

Happily, the factory compensation can be re-obtained by momentarily disconnecting the radio from power. This can be fairly easy done by removing the corresponding fuses, which differ per year and model and can be found in the fuse box in engine compartment, at the firewall at the driver’s side.