Battery power on preamps like Nagra PL-L etc

Has anyone tried connecting a 12v battery supply to the Nagra PL-L? The FAQ section of their website says this is possible:
"However, if you wish to use a battery on your PL-L, you can plug a 12 V battery onto the Lemo connector..."

Battery seems to be the in thing as in Dartzeel and ASR.

Any comparisons on sound quality vs mains?