I would be very interested to know if the sub is clipping due to too much power in, or if it's a signal issue. So first things first, can you get to the wires into the sub? Swap the buggers around. Might immediately resolve it. Subs tend not to like being asked to play backwards!
Next, do you know... is it an IB (infinite baffle) jobbie or does it have a sealed box.
The next thing I'd want to do would be simply throw another similar specification sub in there and see if the issue persists. So if it's sealed, something like a JL Audio 10W3 is likely to be of a similar specification (Or indeed just one of the better Alpine or JBL 10s )
If that new sub sounds similar, you could be looking at a few things. Signal interferance, the "sealed" box not being sealed properly (if it is) or a number of other things.
I would have to hear it though to have any ideas.
One person not too far from you mate who I'd give a very strong personal recommendation to get to the bottom of this is Lee at PlatinumInCar, Coventry. Book it in and tell him you don't understand why the subs sound so poor. See what he thinks. He does some seriously brilliant bespoke work and I'd strongly recommend him.
Down my way, the shout out would go to CarAudioSecurity. Autocapital are always recommended but they to be honest are cowboys (IMHO!!!! Libel aside.. ) who simply overcharge due to the penchant for Ferarri to drop their cars there for work
I would never ever put up with poor quality sound but the difference is; in the cars I've picked up I've always expected it - which is why with every car I've ever owned more than 4 or 5 months I've ripped the whole lot out and put my own gear in
One last thought.. Is there really no one complaining about this issue?
In that case can we be sure that it isn't simply a case of whoever is retrofitting these cars with B/O systems doing a crap job? I doubt they come from the factory with the system fitted. I bet theres a UK car audio distributor doing the work. Maybe the other one you heard is shite because it was fitted by the same cowboys. Just one thought.
Maybe get a good quality audio recorder out and upload it on an american based SLS owners forum, see if you get anywhere.
Or for that matter - the arabic enthusiast forums would be a reasonable shout too if need be.