I've owned battery gear, to include a Dodd battery powered preamp and a battery powered supply for my Squeezebox, which I still use. Based upon my experiences, it is my opinion that unless you just have very poor AC service at your home, battery power is somewhat over-rated. The VAC pre I own now is just as quiet in my system as the Dodd. Another thing to keep in mind is that there is a certain amount of "nuisance" that comes with battery powered gear. Batteries do die, sometimes at inconvenient times, and your listeing time is limited. So, if you like to have long listening sessions, battery power may not suit your listening habits. These issues have caused me to shy away from battery power until better technology is available.
It has, however, been my experience that there is a benefit to having the power supply separated from the "business end" of the digital gear and preamps. With the exception of my amps, all of my gear has separate power supplies and my system is dead quiet.