Hi There,
It's you're lucky day , because I'm the chap who was disappointed with the XJR sound!
The problem you describe does not sound normal. The first thing I would do is press the Mode button and ensure that Bass, Treble, Fade, Balance, and Subwoofer are all set to "0" (this gives the flattest sound); there is another setting called "Position": I would set that to "All" for now. Once this is done, re-test the sound and determine whether it happens only with radio or CD or tape, or all of them. If it is only one of the music sources, then there could be a problem with that source (such as a dirty tape-deck playback head); if it is all sources then perhaps the amp is damaged or you've got a bona-fide speaker problem.
If your 2000 XJR is a Select Edition Jaguar, then you're in luck because the stereo is covered under your warranty. If you do not have a Select Edition then you may want to first determine whether there is physical damage to a particular speaker or speakers before doing anything. If you determine that the speakers are damaged, then you could get them repaired at a speaker repair shop--or simply replace them. I would not mix-and-match brands: If you replace the front speakers (unless you're talking only about the tweeters in the support columns) then I would also replace the rears with the same brand speaker.
I happen to love Polk Audio for car stereos, but the best way to shop for speakers is to bring your own music to an auto-stereo dealer and hear your music on various speakers (realizing, of course, that those dealers can only approximate the car environment in the store). I have found that if you like the sound in the store then you will only be more impressed when the speakers are installed into your vehicle. If possible, though, ask to listen to the speakers in your car before they close-up the grills and door panels: this way if you perchance do not like them, then you can swap them out for another brand with little effort (ask the dealer about this beforehand!).
I found just the opposite problem in my XJR: The high-end (15,000 Hz and above) breaks up only on the front main speakers at volumes above 23. The bass sounds pretty clean to me. I am not doing anything with speakers until my warranty runs out.
Finally, bass is nondirectional. This means that anything loose in your car will vibrate to the bass and it may be difficult for your ear to pinpoint the source of the vibration. Make sure that you have nothing in the door pockets or trunk (where the subwoofer is near) that is vibrating and fooling you into thinking you've got a bad speaker.
The speakers should not be difficult to replace--but I don't know that they're "easy" to replace. You should consult only the finest car-audio store for installation because you don't want some 15-year-old kid ripping apart your door panels and then quitting because it's only his summer job...
I hope this helps...
Regards,
Steve