Would the BB be better than the Pogoplug?
As a renderer/player Should be just as good , best would be to compare the 2 and maybe include the Rpi2 in the comparison . I could make it so they all run the same linux software for LMS to minimise differences . Power supply upgrades is a big factor, so the amount of tweaks you do will ultimately determine.
Another option is bypassing usb all together were a number of options are already out for the Rpi for an I2S connected Dac ,
For audio server duty the pogo plug would be better because of the built in PSU w/c supports usb powered Hard Disk.
If you can provide 2 or 3 same usb dacs and cables , I can provide the base player/renderer hardware for a shootout...
We just play the same music on all the players simultaneously, and switch between them using line level inputs of a preamp
I would suggest a base comparison first using the OEM power supplies .
After determining the platform (pogo vs BBB vs RPi) we can proceed with the choice of software. (LMS vs MPD .. others), also wired vs wireless