Spendor, PMC, ProAc, ATC- Sonic characteristics and differences?

Hello-I am interested to learn more about four of the UK speaker companies that I have interest in, but have no way of auditioning; Spendor, PMC, ProAc, and ATC.

I've read their web sites and tried to find various posts and reviews about their products, but it seems there are not too many overall descriptions or comparisons of these four companies and their signature sound profiles, and the engineering approaches to their products.

I'm familiar to some degree, and have owned or heard, some of the other British speakers from Bowers & Wilkins, KEF,  Monitor Audio, and Wharfdale, but the four companies above are quite interesting as they all seem to trace some level of their heritage to the BBC. And three of them also appear to have Pro Audio speaker lines. Are their designs similar in nature? Do they all sound somewhat the same? Do they all have similar levels of quality, or does one or the other stand out?

Please share your knowledge about these four speaker companies and your opinion of their sound signatures and product quality.

Thank you