What is the Sound of Impedence Mismatch?

The insertion of a series resistance in order to achieve higher input Z may be worthwhile, or may introduce some other undesirable nasties & so could be a tradeoff situation.
In order to do this properly you'll need to get high quality resistors & also find a place to interrupt the signal path, possibly at a point where an input signal coax is running from an input-RCA connector to an input board. If you can desolder the coax connection from the input board & solder in a resistor there then that would likely be a good place. Regarding the proper selection of resistors, if you can get the same brand & type that Pass uses in their equipment then the sonic signature would be compatible with what's already in there. If not then make a call to Michael Percey audio at:
http://www.bainbridge.net/percyaudio/

Thank you, Bob. Great info! I think that making this kind of modification is beyond my comfort level, however. Would there be any way to fashion an interconnect "extension" that would add the resistance and let me see, on a diagnostic level, whether there is a significant change in the sound if I increase the impedence?

Two posssible approaches Dan...
A female-to-male add-on extension RCA connector set could be utilized with the series resistance incorporated therein. However just by adding the extension (even without any extra resistance built into it) you would likely change the sonic signature. I suppose that you could listen to the extensions first (without extra series resistance) in order to extablish a baseline, then add the resistors & listen again. But if you're not solder-savy then you would need someone to build this for you & then revise it later.

Another possibility would be to get some inexpensive (disposable)interconnects that allow the RCA connectors to be easily opened up, then add the resistance right inside the connector. Again, listen awhile without the extra resistance, then add it & listen again. Be sure to allow for cable breakin phenomenon too. However by using a cheapie cable, you're probably not going to realize optimum sonic performance either before or after; still you might be able to detect a change? But again if you're not solder-savy then you would need someone to do this for you.

I think the second suggestion is a winner. I'm pretty sure I can handle the soldering, but--and I'm embarrassed to ask this--the resistance should be added to the "hot", right, and that is which, the inner plug or outer jacket on an interconnect? And, are resistors directional?

While I'm at it, I am also embarrassed to discover that I have been misspelling impedance all this time.

Drubin, don't feel bad I misspelled the name of the item I have up for auction, twice! Then spelled the word in the manufacturers name correctly in the text. Talk about embarrassing.