I owned the ST Audio C-port for while.It uses the same converters as a couple of other similar products. I can't remember what they are, but a quick search of C-Port reviews on the Internet will tell you.
Regardless, I had no problem with them. I even sold them to a collaboration buddy of mine when I was switching platforms.
The only caution I would issue is to read the compatability issues in ST Audio's support area carefully. Their stuff tends to be very picky about MB chipsets, IRQs and PCI slots.
Best of luck to you!
-Rick