Thanks for your replies. I am not looking for more inputs or outputs, just to use the better headphone preamp as a second "opinion" while I am mixing. I currently have something in the SPDIF output on the 4Pre, so I am exploring the USB route. I am unsure if the SPDIF is the exact same quality as the USB on the M900 and if the SPDIF can be split to use two devices AND retain 100% functionality, latency and quality. I know, everything is complicated!
But, no sense in spending up for a high quality headphone preamp only to connect it in a suboptimal way.