"Did anyone compare?
What was the pre you used with each?
....And what is your impression?"
Yes, I am one of the people who have compared and owned the X350.5 and a pair of X600.5 compared to my MKIII MII.
I kept the Spectrons.
My preamps are BAT 51SE and NAT Audio Symmetrical, both are tube pre, speakers are Analysis Audio Amphitryons, Magnepan 20.1 and coned JM Labs Utopia Novas.
Both X350.5 and X600.5 played my Novas perfectly well without the lack of anything, however, playing the large planars, I was not not satisfied at all with the bass response so I moved up to the X600.5 which generated bit of improvement but not close enough to what I am looking for. This struggle went on for over a year.
The Spectron played my Novas effortlessly as if the speakers did not exist.
In the case with both planars, it is music like never heard before, with the bass I am looking for after 25 years.
I know that my Spectrons are extremely superior to any other amps I have owned playing my speakers.
Spectrons presentation brings me to the closest degree of a live music performance playing both coned and unconed speakers.