Are my amp and preamp mismatched?

you can not go wrong if you pick all components from the same manufacturer.

I believe back when the amp was introduced Nelson Pass intended that a solid state preamp, perhaps one of his designs, should be paired with the X-250. That may be why the impedance of 300 kOhms was recommended.

@greycloud, I think you have a good match with your preamp and amp, especially since you are experiencing good low-end performance.

This amp is an early Pass design and has a single-ended input stage as opposed to fully balanced so I think using your preamp is fine. The conversion to a balanced signal is performed at the gain stage which is after the input.
I read the white paper and dont see a problem as long as there is an impedance match. There may be some benefit in noise reduction by using balanced cables.

Someone more qualified in amp design could tell you if there would be a sonic benefit to using a balanced preamp.

I always thought you could not go wrong with both from same mtg. until I owned a McCormack RLD 1 along with a DNA 125 amp.   It was a good sounding combo, perhaps better than my previous Mc C15 driving the 125,  but it just didn't blow me away.  I used the RLD for a few years and then bought a c-j Classic 2SE.   The RLD, was the bottleneck of my system all along.....go figure.    I almost replaced my speakers and fortunately I didn't.....   That preamp made it sound  like I had better speakers, the difference was that pronounced.