I have a few updates here.
It appears that my HTC One X running Android Revolution HD 13.0 (Jelly Bean) has the drivers and loaded modules for snd_usb_audio. And when I plug my USB DAC into it. It recognizes it and automatically directs sound to it. This is good.
I've dived into why it works on the HTC One X and not on my Xoom and come across a folder called /proc/asound
This folder is to do with the ALSA (Advanced Linux Sound Architecture) and when I plug my USB DAC into my One X, it creates a folder called 'Headset' (since my USB DAC is a headset).
I've found folders named "snd_usb_audio" (or similar) in the following folder locations:
I just don't know how to add this gory stuff into another android device. This is what Im trying to do.
Im going to compile a kernel in the next few days to see if I get anywhere.