Are my amp and preamp mismatched?

"The Pass has an Input Impedance of 22k ohms balanced"

This impedance rating is for a balanced signal and I believe your preamp is single ended. The input spec of the Pass for SE is 11 kOhms.
The calculation for impedance matching is 20. IMO, this would be the minimum needed for your preamp to drive the amp with no loss of dynamics at all frequencies. However, peaks and transients in the music have to be considered, but they are of such a short duration that clipping by the amp may not occur.
  The reason I use the impedance calculation result of 20 rather than 10 is to take into account transients and the impedance of low frequencies.

The X-250 is not a fully balanced differential amp, so it accepts a SE input and converts it to a balanced signal. 

So how does it sound? Ever notice any clipping? I'll bet the two components work well together.

Thank you for the reply. No clipping yet. The sound is solid, especially the bass. The music definitely has more presence than my previous tube amps.

How do you think using a balanced preamp would change things?

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

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.