I have a couple of questions about the pico power that hopefully Justin can answer.
1. Is the background hiss level for sensitive IEMs less than that of the Pico Slim?
It depends on the gain setting. In low gain yes, in high gain no.
2. At the lowest gain with sensitive IEMs listening at low levels, will you be listening above the volume pot's unbalanced portion?
Yes. i hand test every volume pot that is used, and only take the ones that have good channel balance at low levels.
3. Do the two 9v batteries have to be equivalent, what happens if one dies first. I guess the real question is are the batteries connected in series?
Yes, they need to be the same. The batteries are in series. The amp will not function with a dead battery.
4. Are there sound quality differences if you use different voltage 9v batteries, in the nimh variety they exist as 7.2v,8.4v, and 9.6v variants nominal voltage.
The higher the voltage battery, the more power you can get out of the amp.
5. What is the max voltage the batteries can be? The voltage can reach 11.52v each for a fully charged 9.6v nimh version.
i wouldn't go any higher than that
6. Are there sound quality differences using batteries that can deliver more current (li-ion) vs those that can't (carbon-zinc)
if you are trying to drive a very inefficient headphone such as the hifiman he-6, the li-ion may be better. they also maintain a more stable voltage thru their discharge.
7. Does sound quality degrade as the batteries run down?
You would lose max output power
8. Does it have a mechanism to let you know it's time to replace the batteries before just hearing distortion?
The power LED will turn off when the combined voltage is below ~12V