OP,
Unfortunately the percentage thing does not work because we all have vastly different weighting mechanisms and values. But I can put it this way the SP9 was a notable upgrade from the 8, the Ref 2 notably better than the 9 (there was some controversy with the REF 1… the REF3 was an improvement over the REf2. That is the way I would look at it.
I have most recently owned a REF5SE and upgraded to REF 6SE… with a notable improvement.
Another way of look at it you are talking about the SP8 first introduced in 1981 versus 2005… with continuous improvement. During this time ARC made very significant gains in detail and noise levels… keeping midrange bloom and musicality and approaching neutral from the tube side… while solid state was get warmer from the brittle screechy side. If I had it on a 1981 design preamp, it would be a tube preamp, but I would prefer a tube preamp from 2005.