The Response 2 is between the Sig Tab 50 and Response 1 SC in efficiency. The R2 will do OK with a Nait and will only get better with better amps/sources.
The obvious difference between the original 2 and the 2s is the port placement - the original 2 has the port in the front and the 2s has the port in the rear. Mine are the original 2 variety and the benefit is that I can get away with placing them near walls/corners without inducing overbearing boom - BUT I don't have CDX or better as a source, so my system sounds lean compared to a CDX based or higher system.
I don't know used prices in the UK, but after converting to US Dollars the price doesn't seem too far off - especially since the stands are included. Watch out for shipping costs though - the stands are h e a v y...
- GregB
Who still has Response 2 speakers but is not using them at this tim