Unfortunately the information provided by PreSonus is not overwhelmingly much and nothing will answer your question. I don't use Pro Tools, but I read here and there something. I think the first question must be if you use the 32 or the 64 channel version of Pro Tools. If you use the 32 track version, the USB routing of the mix bus must stay in the 1-32 range, hence the Tape In/DigRet stereo input channel must be set to the corresponding USB sends. If you use the 64 track version, the USB 37+38 routing can be used. I don't know how to route USB output sends to the mix bus of Pro Tools. The song templates of Studio One have all this integrated and ready to use. Studio One also allows to 'patch' USB output sends. If I want the mix bus to be sent to USB 1+2, I just change the setting in the USB output routing of ths song. It's not obligatory to use Tape In and USB 37+38. This is just how the Series III mixers and Studio One were designed to work together.