Use reputable hardware vendors.
Folks,
You managed to get Mac Pro and the best you can do is to plug in a cheapo audio interface? Ouch!!!
I bought used MOTU 828MKII few years ago - zero problems on Tiger, Leopard & Windows, period. Not to mention its phenomenal performance all the way to 96/24 with smallest IO buffer (32 in LogicPro). It is not that expensive, but once you get your hands on any MOTU - you would not want anything else...
I also have E-MU 0404USB ($199)- no any issue on Tiger and Leopard, not as fast as MOTU on Mac (probably as fast as FW discussed here), but on PC it will leave in dust most of the lower-end FW ($300-$500) like Echo, Edirol, Firebox, M-Audio and etc.
BTW, poorly written 3rd party device drivers (sometimes a device firmware) are source of 99% of KP. Being there, done this - one little typo or error and kiss goodbye.
Just get stuff that works, if you can...