http://www.diyrecordingequipment.com/store/product-category/preamp-input-impedance-control-kit/

A thing that may be of innerest. The machined aluminium chassis is probably the 'spensive part. I'm betting if you have your own hausing für it- or no hausing whatsoever, it could be even cheeper. But in a nice container it's twenty bucks and change.

Honestly with my kind of patience I'd just twist it into some arbitrary position

The impedance on my ART DMPA, I pretty well have sorted for various mics. It's not a tweaky thing at all, it's more a "keep playing for a minute while I turn the knob" thing and just earball it. "Okay, sounds good right about here". If you're just throwing shit up and going, down all the way for 57s and up all the way for condensers; ribbons and other dynamics may vary.