Glad to hear confirmation that the SBM-1 will indeed work with the 680.
Just for clarification, the Sony is doing the dithering and truncation down to 16bits, not the Tascam. The SBM-1 output is 16bits. The application of noise-shaped dither to the 24bit internal output of it's ADC prior to truncation to 16bits is the Super Bit Mapping part, which lowers the noise-floor and makes for an "effective" range of somewhat more than 16 bits in most sensitive frequency range of our hearing, but the actual output format is still 16bits. The Tascam is going to either write a 16 bit file if that's what you tell it to do, or a 24 bit file if you tell it to do that instead, regardless of the bit rate of the signal being feed into it's SPDIF input (only the sample rate needs to match). The resulting recording on the Tascam will contain the same 16 most significant bits either way, stored in either a 16 bit or 24bit file.
If you feed an actual 20 or 24 bit signal into the Tascam, it will record all of that information completely if set to write 24 bit files. If set to write 16 bit files, it will truncate the least significant bits from that input, without using any SBM-like noise-shaped dithering process.
There may be some way to mod this thing to output the 24bit file before the SBM noiseshaping, and I could be wrong but I don't think Oade offered that. It isn't one of the three SBM-1 mods that were once described on the Oade website at least. If so, it would no longer be doing any SBM'ing.