I want to get 4 inputs simultaneously through my Mbox 3. So my only option (besides upgrading to PT9 of course, but I'm hoping the 2 S/PDIF inputs will be cheaper) is using the 2 on board and getting 2 more through the S/PDIF input. Never used S/PDIF before so I hope that's right and it will work. If so, what's my best cheap option?