Not sure if in reference to preamping. But I'm found this discussion on preamping and damping factor:
http://www.hifivision.com/amplifiers/12302-simple-question-about-damping-factor.html
Even the Wikipedia article on Damping Factor talks solely on how the amp interacts with the speaker cone or diaphragm, and how well it controls the movement of it. It seems like the only problem with lower ratio of preamp output impedance to amp input impedance is how it affects the noise floor.
And if you're talking about amp to headphones, then you're absolutely right on damping factor being a concern, though I'm pretty sure the MiniX is designed to drive 4 ohm speakers, so the 38 ohm HE-500 shouldn't be an issue with damping factor.
As such I have no problems with bass after adding the Quickie to the chain, even though it's 2K ohms and the MiniX is 10K ohms. In fact, I found the bass to be more refined and better. The noise floor did drop from 12 o'clock to 10 o'clock on the MiniX after adding the Quickie, but I never have it go past 9 o'clock, so it's not really a problem.
Also, I've been told the PJCCS upgrade on the Quickie raises the output impedance from 2K to 4K ohms. Probably not a good idea for pairing with the MiniX.