The biggest impact is that you're in a car.  Not only are the acoustics of a
car terrible, not only is it impossible to place speakers sensibly, but
there's a load of mechanical noise from the engine and suspension and wind.

The usual solution to improving sound quality in a car is (a) lots of
soundproofing to keep the other noises out (Jaguars do well here), (b) lots
of speakers (which tends to be what the high-end sound systems) to get round
the acoustic deficiencies of the cabin, and (c) turn the volume up because
louder sounds better.

Processing the sound source also helps - compressing the volume range, and
emphasising frequencies that otherwise get masked by engine, suspension and
wind noise.  I have some old sound processing software for copying vinyl to
CD that comes with a "car" filter.  IMHO you get some of the same effect
with MP3 compression - that removes "background" sounds to reduce the size
of the file, and I find the results are more listenable than the original in
a car.   But it's not HiFi.

I'm not sure what's the standard sound system on the XJ8 you are looking at,
but it's quite probably Alpine.  Don't forget that luxury cars start at a
higher level, so their standard sound systems are lesser cars' premium
systems.  On the current XF, for example, the premium sound system is AFAIR
a B&W setup at several £1000 extra.