Older AI Head - Impedance Questions

I have two AI setups - the newer one is a Series III Ten2 and I have no questions about that. The older is an original series head plus a Ten2-EX. My question is about the older head

There are two inputs, low and high gain. The high gain input has a two-position impedance switch, low and high. My question:

What impedance setting is likely to yield a flat, or at least the flattest, response with different common inputs? I ask because I use both my AI setups for everything under the sun.

Specifically - and this is a bit off-topic, I realize - is a plain, old, passive electric guitar or electric bass, whose low output suggest using the high gain input, likely to yield the flattest response using the low or the high impedance switch position?

And how, and why, would I want a different setting for my SixRounds piezo transducer?

Thanks very much in advance, and any novice-friendly electrical education would be appreciated, too.

-S-