Conventional wisdom, is that the Ref 5, is'nt a great advance on the 3, but the 5SE was a big step up. The logic of that is, if the 5SE is to expensive, which it would be for me, then choose the Ref 3. I have to say, I have heard the 3 and 5 SE, extensively, but not the 5.
The alternative would be the LS27, which I have heard against the 3 and I think, is better than the it. That can be bought second hand at a comparable price to the 3, I believe.
Having said all that, I bought the Modwright 36.5, which I consider very well made and better sounding than the Ref 3, at a cheaper price.
Returning to the Ref 5, I personally can not understand, why a large reputable company like ARC, keeps producing Pre and Power amps, which have to improved by an SE upgrade a short time afterwards. I understand to an extent, with my Ref 75 Power amp, where the change was to accommodate the better KT150 tube. This doesn't seem to apply to the Ref 5. A cynic would say, this is to make more money on upgrading.