I plan to get the SP to use as an interface as well, and I wanna know if I could bypass the built in mic preamp, and use a different mic preamp? Would I be able to connect a mic to say a GT Brick or RNP connected to the SP555?

Well the only ins are the 1/4" on the top and the rca on the back. The sound would technically still be processed by those inputs since there's no way to bypass. You'd think both of those pres would have a USB out on them since they're so pricey but since they don't you're gonna have to add some other piece of hardware in the signal chain no matter what. I haven't used the preamp much but I haven't heard many complaints about it either. It's a little on the quiet side