I'd set the Lucid to output 16 bit dithered, then run the signal via SPDIF to the DAT.
DAT recorders that applied dither on input (eg, the later Sony PCM R series with Super Bit Mapping) did so on the A-D side, not via digital input.
Originally Posted by johnnyv
You are never going to really "hear" a difference between 16 and 24 bit recordings.
While the difference may not be audible as-is, the distortion from truncating without dither can easily accumulate, and at levels higher than dither (and of course being uncorrelated with the signal). Nothing to lose from dithering, and it does preserve low level detail in the 24 bit source.