Preamp: adjustable impedance + GAIN

Well, I recommended this once before, You didn't say anything in that thread, but it answers what you are looking for.
http://www.coda-continuum.com/product/index.php

I've never seen a preamp with adjustable input impedance.
Coda is a group of engineers that were part of the design team for Nelson Pass at Threshold. They do have a different flavor than Nelson's current stuff, but these guys are very competent designers. The 07X preamp can hold its own with the best solid state. Might not be your favorite flavor, but its worth checking out and they have auditions, if you don't like it, it will only cost you shipping cost to send it back.
I hope this helps,
Tim

Olympia LXi

Can drive any amplifier, with ease.

From its maker

Good Listening

Peter

I am not aware of any preamplifier with adjustable input and output impedance. Not saying no such device exists, just that I've never heard of one. Other than that, the new Esoteric preamps (C-03x and C-02) may satisfy your other requirements (the gain of each input can be adjusted +/- 18 dB), except that they use sealed sub-chambers for the internal power supply rather than a separate power supply box. But be prepared to pay large!

You just want the lowest output impedance you can get in the preamp.

What is actually just as important is not so much that the output impedance is low, but also that it is the same at 20Hz as it is at 1KHz or 20KHz.

If balanced, best results will be had if the preamp ignores ground at its output- that is to say that the output occurs between pins 2 and 3 of the XLR, and pin 1 (ground) is merely used for shield.