How old are your Powerex 9.6V batteries?
Are you measuring their voltage with them in an open circuit condition (not powering anything) or while under load?
As the batteries age, their internal resistance increases, which is essentially what causes them to no longer perform correctly. Measured directly out of the charger in an open circuit condition, and old battery with high internal resistance will often indicate correct fully charged voltage, but as soon as it is put under load the voltage will drop very quickly to questionable levels. Figure a way to test voltage with the battery in the preamp with the preamp running. Alternately you could measure the battery's voltage with an appropriate sized load resistor across the terminals.
This may not be the problem in your case, but it's the first thing that comes to mind, and I've experienced it myself numerous times. It was confusing until I understood what was going on.