Of all the great traditional British names, hardly any of them are manufactured in this country any more as it's far cheaper to ship the designs to Eastern Europe or Far East for them to make. I used to have a pair of fine British Tannoy speakers that were manufacturered in Hungary.
Yes, a good solid slab underneath a sub can tighten up bass response a good deal. I have also heard of people putting slabs on top of the sub as well. I think that one foot thick is a bit OTT but a solid paving slab should show an improvement. It can always be covered with some fabric to make...
My philosophy has always been that a poor subwoofer is worse than no subwoofer at all. I have never heard of any of the makes that you listed as they are not available in England but you can't get a decent subwoofer over here for less than $500. Specs are also misleading and you shouldn't...