I’ve repaired a few Naim 250’s, the best was the first model with regulated rails back in the 80-90’s and they weren’t that good, they all oscillate at a drop of the hat, don’t think it would take much to beat them. If fact Naim dealers warned against putting them together with Quad ESL’s. And in the 80's Tocord speaker cable made them blow their guts.
Cheers George