This thread was supposed to be about subwoofer suggestions not the tweeter and has gone off topic. I should of never mentioned the tweeter in the first place...
I can understand your frustration but really some of us think that adding a sub may not be the solution to your problems.
PSM1 is right about the prospect of the sub being abused in the way your Wharefdales may have been. Hugh9191's suggestion of working towards a system that would suit you better will actually save you money in the long run by preventing you from heading down a dead-end road and ending up with something that may not be the full answer (i.e. xr-5000 + sub).
According to everything I've read (I've not heard them), these xr-5000 speakers have enough bass impact, bass extension and volume for domestic parties – let's face it, people aren't buying these for their midrange transparency and spatial imagery! See this earlier thread. Note the specs on bass extension - should be more extended than your original post suggests. In room, this speaker will likely reproduce a decent level of bass down to around 30Hz:
http://www.avforums.com/forums/speakers/983260-wharfedale-xarus-5000-pair-155-a.html
I don't know what music you tend to play when you party but if it's the usual clubby house type stuff and pop, then there often isn't much sub 40Hz bass present. As I understand it, rythymn-driving bass is usually in the 40 to 80Hz range, which is what the xr-5000 was designed for. Perhaps you want more 40 - 80Hz bass rather than out-and-out extension? I guess a subwoofer would still oblige. If you go down the subwoofer route, earlier posters have made recommendations.
From afar, we can't be certain, but if the Wharfedales need to be driven too hard to get the sound you require, then PA is the way to go. PA gear is more robust and often has in-built protections that significantly reduce, but cannot entirely eliminate, the chance of equipment damage. And these days, decent PA speakers will sound at least as good as your xr-5000s.
Speaker protection devices:
http://www.stagebeat.co.uk/Monacor-DSP-1-Speaker-Protection-122850-100388?source=froogle#longDesc
Not sure if suitable. You'd need to check out the power handling of the tweeter only (ask Wharfedale?) and if it's 20W or more (often they're less), this may do the trick but one of these would need to be wired to each individual tweeter.
D-fend
This looks to be the dog's. Don't know of any retail outlets that sell it and no idea of cost.
Typical £10 – 20 protection devices simply block DC from entering your speakers – they won't stop overdriving and clipping.
Probably cheaper to buy a cage and put the amp in it - or maybe your guests?
Good luck whatever you do.