UsB host on a motorola xoom
i noticed all the rooted MZ601 roms, have poor OTG support in the kernal, or next to none, i had the same issues with eternal USB DAC, not been recognized since the 4.0.4 stock ROM, after spending weeks , blowing images, roms, went back to the stock ROM 4.0.4, seems OTG support is not on their high list to things to do, only external USB hard discs.

the USB DAC is only 4 months old, so its not some special kernel patch, as the 4.0.4 is over 2-3 yrs old and worked, try going back tot the stock rom.