I should have mentioned that I have been using a battery powered Nixon DACkit for going on two years, and it sounds just great. Liquid, fat, immediate, all that stuff, and goes hand in glove with the batTery amp. I don't think things get a whole lot better for Redbook, in fact. YMMV. Scott said this iteration was about his favorite DAC design in all his years of building stuff. Less parts than the 47 Lab 4715, and supposedly equal or better overall sound, subject to interpretation, and opinion, of course. The 47 Lab unit is thought of as "Class B" by the Stereophools. John Atkinson, whose ears I do trust, liked it ALOT, calling it "more musical" than other, higher minded designs he had heard.
I didn't detect a great difference when using different transports, but I don't consider myself one to vivisect the music. I think a USB based bit perfect datastream might change its sound, but I personally would start with the Sony portable IF it has an SPDIF output. In my very limited purvue, I think the best battery powered transport would in fact be a notebook computer with an outboard soundcard. Then a battery DAC and ClariT hooked up to the Biggest Horns, or open baffle rig you can fit into your house.
Transport mechanisms have been pretty well thought out at this point, I would imagine. I'll wager the Sony is a good bet for an experiment. Then find out if it can be re-clocked maybe.