I then went out and recorded something as a real-world test about two weeks ago - a national rock act performing at a local music club. It was not obscenely loud, yet definitely above the ear-plug threshold SPL level for me, and probably typical of the material most members are recording around TS. Average SPL for this was definitely higher than most of what I record these days, which fit my intent to check for low frequency distortion in the resulting recording.
For this I set the rotary gain on the 4ch-CA-UGLY to it's lowest position (0dB, fully counter clockwise), whereas I typical have it set to the +30 position for chamber music and other acoustic material, or +15 for larger ensemble classical or small combo amplified jazz material. However, keep in mind that the actual gain through my preamp is always less than what is nominally indicated by the rotary gain switch setting, due to how I have the individual channel gain-trims set. The trims are all set to less than max trim positions, partly to balance the level of all four mics in the arrangement I use them, and party to achieve the best match with the input sensitivity of the DR2d. I've set the trims in such a way as to maximize the useful range of material I can record by simply setting the rotary gain switch appropriately. That way, I can use this combination of gear to record the loudest music I'm interested in at the 0db setting, as well as as quiet ambient nature recording at the +35 or +40 settings, and all the material I typically record at the available settings in between those extremes. So in my case the true gain through the preamp when set to 0dB is actually less than unity. In other words, the preamp when set to the 0dB rotary gain setting is attenuating the signal prior to reaching the DR2d inputs, which is appropriate given the high sensitivity and output of the 4060 and the ~2V max input level capability of the DR2d.
The good news is that listening to the recording I can detect no obvious low frequency distortion. I've not visually examined the wave forms, but am personally satisfied that the 9.6V Powerex precharged NiMH is the most appropriate battery currently available for this preamp, providing sufficient voltage/current capability to avoid the distortion problem I was having occasionally using a standard alkaline (Duracell) 9V, at run times up to at least ~10hrs before supply voltage drops to less than 9V.