The headphone jack on the 671 IMHO isn't very good. The built in amplifier isn't very loud, and it has a high noise floor. One of the only things I find that I don't like about this unit. Use the RCA outs if you can.
Why would the recorder you're giving the patch to have to be 24 bit? You're feeding it an analog signal, for it to run through it's own ADC. AFAIK, there is no difference (from the patching recorder's point of view) between an anlog signal coming from a 16 bit device and an analog signal coming from a 24 bit device. Analog is analog. Am I wrong?