Thanks for your experience.
I have found, by accident, that the B+ can go QUITE low and still "work." But distortion is fairly high and the sound quality isn't very good. In general I prefer them at about 6v or more each, so 24v B+ minimum. But I often inadvertently listen below that because two of the NiMH batteries I often use seem to drop extremely quickly, well before I am ready for it. So there is a short period when I'm running at well below 24v. In fact, one battery seems to read at -3 or -4v or so...odd. As it happens, this can occur when batteries are used in series like in Quickie. If one drops to 0v but current continues to flow, it is possible (according to a source I've found) for current to flow in the opposite direction (particularly, I assume, if as in my case the positive side of that battery is set to the ground potential in the circuit and thus provides a negative voltage to part of the circuit). This is not good for the battery, so you should change batteries well before this happens.
As for the D-cell filament batteries, I've found I can go as low as 0.9v or so if I want to. Below that, the loaded voltage (voltage when turned on) is actually more like 0.5 or 0.6v and that is just too low. Distortion isn't terrible, may not be noticeable, but it's just not worth it. Oddly, above that point it seems like the loaded voltage isn't a lot lower than the unloaded voltage. So there seems to be a magic point at which the batteries just truly run out of steam.
