What is the Sound of Impedence Mismatch?

I've read that bass rolloff (happens due to interconnect capacitance) can be caused by such an impedance mismatch where the pre's output Z is too high for it's PA load, so you might have such an issue Drubin.
Have you experimented with different upgrade AC cords though?

Dan you are a dreamer indeed, but no fault with that! Your suggestions are really terrific. I think for now at least the closest that we'll get to "guidelines" are to be found here on these forums (try this - try that - see what happens - scratch your head some more). The funny thing is that a particular setup for one system may not be at all appropriate for a different one. (One person says that this XYZ cable is great, yet another person is totally dissapointed).
The "test disc" idea seems quite worthwhile; when are you going to get started? I'll buy the first one off the press..

Keis are you familiar with the concept of LCR cable-reactance-induced frequency response rolloff? I cannot at all agree with your statement above.

Poulkirk your above treatise is very nicely explained & prompts me to clarify. Loading & clipping is of course a valid consideration. But it is not *the only* consideration of impedance mismatch, which is more generally manifested in freq. response problems than in clipping distortion.

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/

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.

Dan yes: the inner conductor. Direction of the resistor is not an issue.
Be aware however that the input sensitivity of the amp will be reduced somewhat, requiring a bit more drive level from the preamp source component.