The problem is now solved. It turns out that plughw:1,0 directs output to the first two channels of the card. However, the "IEC958 output also on ADAT1" amixer option was switched on by default, directing output to the IEC958 (s/pdif) channels to ADAT1, instead of the output of ADAT channels 1 and 2. From comments in the source code it appears that channels 17 and 18 are devoted to s/pdif. When this option is on, I assume that output channels 1 and 2 go nowhere, though this hasn't been confirmed. The solution was to turn off "IEC958 output also on ADAT1"; thereafter, aplay -D plughw:1,0 file.wav worked, that is, produced output on ADAT1 (channels 1 and 2) as desired.
