There are many of us here who are running V3 > MicroTrack.
With the latest firmware (v1.1.5-BETA), the MicroTrack will record at 24 bit via the S/PDIF input, although only at 48 kHz. 24bit, 96 kHz is coming soon, hopefully.
as has been mentioned, the V3 has many digital outputs. but you can't send both a 16 bit signal and a 24 bit signal. all the digital outputs are either 24 bit or 16 bit. you can send a 24 bit, 48 kHz digital signal to a DAT deck, and it'll truncate the 8 least significant digits, and only record 16 bit, 48 kHz. truncating probably isn't the best way to run a DAT back-up. if I were you, I'd run analog out of the V3 into your DAT deck line-in. although your DAT deck probably doesn't have as good of an A/D, you are still taking advantage of the wonderful pre-amp of the V3, and you won't have small digital artifacts left in the recording from the truncated signal.
that's the short answer. search around a little, and you'll find PLENTY of information. this has been covered a lot in the past.