The PMD671 and PMD670 only output digital when taking in a digital from an A/D like a V3 or MiniMe. When running these as front ends, with or without outboard preamps, they do not output digital. I am not sure what the Tascam HDP2 does with digital signals, or the PCMD50 for that matter.
This basically means that you could face constant roadblocks with not being able to be fed a digital signal, except from units like the Sound Devices 7XX series using the correct adapter to connect to it. At least, you need a recorder that can handle analog input signals as well as digital, and be compatable with everything.
If I was going to get a deck for patching, I would get an R44 because it has full size analog inputs and outputs, and coax in and out, it's smaller than a P2 or 671, and it could one day serve as the foundation for buildling a full rig. You could even run your own matrix recordings without owning your own mics, by taking feeds from the SBD and someone else's mic rig and only have to carry the R44.