What's The Big Deal About Battery Powered Preamps?

If the preamp is otherwise well designed, a battery power supply lends a very low noise floor. The importance of low noise at the preamp stage cannot be overstated - as noise and distortion are additive, and the amplifier magnifies the input signal from the preamp, preamp noise and source noise are particularly deleterious.

For preamp I agree it's great. The downside is when the battery runs out meaning that you should have either an extra battery just like for your DeWalt cordless drill or wall socket option.
I DIY-upgrade now my 30y old Classe 30 to work off the 24V automotive battery. A power 2-way switch will power up the battery when preamp is OFF and power ON safe battery charger. Contrary when the preamp is ON the safe battery charger is OFF. For listening session upto the whole day and night of charging will be plenty enough to run on pure battery power.

Mitch, good question. I'm running a dac and integrated amp off a 12v car battery and the experience has been nothing short of an audio revelation. My source is a laptop which I run off the battery so the whole system is off the grid.
The amp can only output about 10 to 20 watts from the 12v, but the power delivery is huge since there's a huge power reserve in the big battery. The sound is crystal clear with huge authority & control. You need a pretty efficient speaker. My speaker are horns. You could bring in another 12 volt battery and double the voltage to 24 which would double wattage to 20-40 watts, and on & on...
The dc power is immediate - there's no ac to dc conversion so there's no need for the big and noisy transformers (this is where much time and money goes to developing hifi gear-quieting the power delivery through the transformers). I'm not too knowledgeable on the technical side of this and hopefully another member can comment on the mechanics. I'd recommend giving it a try.

Battery power (DC) provides 100% effective filter against AC hum. Also provides a stiffer current and prohibits any form of AC contamination.

Go all out with a 12 volt system. Red Wine amp and preamp, battery DAC and MacBook Pro. Get a couple of solar panels and whatever batteries you need to backup your units' onboard batteries for lengthy night listening sessions and you'll be insulated against power outages.

Car stereo would also work.