I am going to try to answer several questions all at once. These two tests were conducted at least 2 to 3 years ago. I had heard the X350 and X350.5 compared side-by-side. The speakers were some big Avalons. Front end was a Sony SCD-1 feeding into a Sonic Frontier DAC 3, then into a ARC Ref 2 mk I.
The X350 sounded sterile in that system, had the very typical SS sound that people gripe about. Whem comparing to the X.5, the differences become more prominent. The X.5 had a warmer midrange, but at the same time better rendition of the bass, also deeper. More air on the top, but doesn't not feel so edgy. All the panelists was surprised at the amount of differences we had heard. We thought the differences merits more than a .5 upgrade status.
Some time later, we did a comparison between ARC Ref 2 Mk I and Pass X0.2 with the same set of panelists. At this time, my friend had kept the X350.5 and sent the X350 home. He had these two preamps for a while, so he was deciding which one to let go. We went back and forth. The X0.2 actually had a warmer, more tube like sound than the Ref 2. The Ref 2 was more extended at both ends. (Shouldn't it be the other way around?)
At the end, the Ref 2 won by a slight margin. I was able to convince the others that the Pass remote just sucks. I am fairly quick learner with gadgets, but I cannot figure out how to use the damn remote. Second, I think with a set of good NOS tubes, the Ref 2 would outshine X0.2 in the midrange department. So at the end the Ref 2 Mk I won based on "potential". But we could live with either happily.
FrankC