I did a great deal of testing last night with the 168rc, a Sonorus StudI/O, and a 24bit 48khz WAV file. I fed the signal through the 168rc's Digital IN A1, routed it directly out Digital OUT A1 and recorded the result via Wavelab. The 168rc took the 24bit data and truncated it to 20bit. I confirmed my setup by taking the lightpipe cable and looping it back (OUT-IN) on the StudI/O and got an exact 24 to 24bit copy.
A technical representitive at Sweetwater Sound told me that ADAT units made a couple years ago, and you can tell by the ADAT logo on the box (which the 168rc has), will only allow up to 20bit data. Looks like he was right. I was hoping to use the 168rc in conjunction with a 24bit converter box (like the Tango 24) and use the 168rc to mix down the tracks. Looks like this wouldn't work, but would be OK for a 20bit converter box.