Thanks for the update Tim. Remember that the mini's have a core-audio soundcard incorporated. It could be that the external soundcard you are trying to use does not yet support intel-based OSX, hence the pops. Surprisingly, the built-in soundcard on all macs is capable of low-latency, low-cpu performance (something that is so unheard-of for us PC users, that many people don't even think to try it).
So, if you'd rather be running OSX, it may be worth your while trying out the onboard soundcard with RAX. Remember the mini sports a digital output (just plug a toslink lightpipe into the seemingly analogue output).
good luck [[;)]]