Preamps need internally high voltages. For use from a battery, DC-DC conersion is needed.
DC is practical, but exist any Dc powered preamp at the level of the big AC guys?
If you want a critical recording gear, would you go for an AC preamp+ a pure sine wave inverter, or is any DC powered one who have nothing to envy to the AC ones?
Here is some perspective from recent personal experiences.
The need for 'high' voltages (higher than batteries can easily supply without resorting to DC boost circuitry) or not in a pre really depends on the LINE input signal voltage (dBu) requirements of a particular deck model.
Most decks require more or less ~ 1.5 volt rms (~ +3 dBu) maximum line input level for unbalanced input, or +9 dBu for balanced line in.
For these decks, I make several models of preamp running WITHOUT DC-to-DC boost. This insures having lowest levels of circuit noise as batteries are inherently VERY low noise suppliers of power, and are way quieter than most any AC power supply source. These miniature preamplifiers produced have amplifier circuitry with full rail-to-rail output ability, so using only two AA 1.5 cells gives a true bipolar +plus/-minus 1.5 volts DC supply, and this is the maximum output swing voltage of the preamplifier. These have great advantage to run a VERY long time with two AA L91 type batteries running one model for 200+ hours, and simpler preamp running 500+ hours.
An example of a deck needing very high voltage line inputs is the SD 744 model that requires up to +24 dBu line signal.
A preamplifier suitable for this deck requires having +plus/-minus 15-18 volt supply rails (30-36 volts total). So little practical chance for running directly off batteries. Instead, usually a 6-12 volt battery is used to power DC switching boost supply, and then this supply also requires very careful noise filtering engineering so not to contaminate the audio signals.
I also make a twice size 4 channel preamp model for 744 type of deck that's unique in quietly running the DC boost supplies 15+ hours off 4 D alkaline cell power sled.