ooh, ooh, sir, I know this one!
I went through this earlier. I sashayed into my local Tannoy dealer clutching WHF and asked for a set of 5 star Rev Sigs. We had a discussion because they didn't have any stock, but did have the regular revolutions which were, as you say, a lot cheaper.
So I retired to consider the options.
And then an email from my man at the shop:
"We had one of the sales managers from Tannoy UK in store this week and we discussed the revolution speakers, basically the only difference between the revolution and revolution signature series is that the signatures have an earthing post, different cabinet shape and they are an English vs Chinese build. However he also said that in A/B comparisons they sound very similar and you would be better off with the straight revolutions. The only situation where the earthing post on the signatures is useful is when you get a lot of static noise at low volumes."
Which I was a bit sceptical about because the differences seemed quite significant to me, and I also remembered that when WHF tested a regular rev it only got about 3 stars (can't remember which actual model - maybe a dc4).
So then the shop actually got a rev sig and the next email:
"Upon hearing the Tannoy Signature Revolution DC-6T's vs the Revolution models it became very obvious that the Sig Rev's are a far superior speaker, so I'm not sure what the UK sales manager was talking about. We don't have them in stock currently but I can order a pair down from Auckland and have them here and run in by Friday for a demo if you are interested."
So I bought the full Revolution Signature DC4 fronts, centre and DC4T surround. I already had a good Velo sub waiting in the wings.
After some initial uncertainty, I am now very happy with the sound which still seems to be improving each time I listen.