Since you are using two 12V batteries, did you consider a charger/load switching scheme that seamlessly switched a charged battery to the load and the charger to the other battery?
No, did not consider that. But I can definitely see the benefits of this approach. It wouldn't take that much of a design change from our current version to accomplish this but it would indeed require new circuit boards etc.
By way of some background, the PS12B was originally conceived as a 24 volt battery PS that would serve as a split voltage (+12/-12) battery power supply with virtual ground for powering of a solid state buffer with the ultimate in low noise DC power. This is still in the works and with some minor part changes and reprogramming the PS12B can become the PS24B. In the interim we decided to simply run the 2 12V batteries in parallel and released it as a nominal 12 VDC power supply that could be used with our existing LDR passive preamps.
Would the LDRv2 benefit from low series effective resistance ultracapacitors provided power-either alone or in parallel with the batteries?
In my view there would be no advantage to this with our LDR preamps. While our current passive preamp design benefits from a quiet (low noise) power supply (used to drive a pair of op amps) they do NOT use this power dynamically. By that I mean the power supply is used to do some very modest amplification/buffering of control power signals that are themselves constant DC (i.e. static) at any attenuation point and only change briefly when volume level changes. This type of system would not benefit from improved dynamic response which is what caps are usually there to provide in conventional audio preamps.
Every now and then someone will point out that our preamps are really not "passive" preamps because they use a power supply. To which I'm happy to concede that they're probably more accurately described as "actively controlled passive preamps". The key distinction between our passive preamp and a true active preamp is the power supply is NOT electrically touching the audio signal, plus there's no gain (amplification) and there's no (impedance) buffer. The power supply does statically control the resistance levels of the LDRs but this is done optically with photons (LEDs) and not electrons.