Pre Amp for Pass Labs X150.5

I use an Atma-Sphere MP-1 with Pass Labs XA-60.5 with excellent results.

I believe in general, you want to consider preamps with less than 200 omhs output impedance to match with Pass Labs if you're going to run single ended, and 600 ohms or less if you're going to run balanced.

30 kOhm is balanced.

20 kOhm is single ended.

The Capri would likely work very well. Buy one used and try it. Re-sell it if it's not to your liking.

It simply means you'll turn the volume knob farther on the 4dB preamp, and you may have smaller steps in volume versus the 14 dB preamp. In other words, the 14dB gain preamp will get louder faster than will the 4 dB preamp, and the 14 dB preamp may get too loud, or be too loud, at a low volume control setting.

30 dB gain on an amp is toward the high side from my experience.

Talon4 described it pretty well.

Impedance mismatch generally results in early bass roll-off.

Gain mismatch generally is an issue if the preamp has to much gain because you may have too much system volume with the preamp at it's lowest setting. Low preamp gain generally just means you have to turn the volume knob farther than you think is normal, but it usually doesn't affect the sound in a negative fashion.

Ability to listen at low volumes late at night is more a factor of speaker/amp match, and good power conditioning contributing to a low noise floor, IMO.