I have resolved this issue with the help of Jim Hillegass and miniDSP's Tony Rouget.
It turns out that, despite the multitude of options and variables, the critical factor is the number of channels on the recording. If there are 6 channels, all's well in PCM and in DSD-DoP. If there are only 5 channels, i.e., there is no LFE signal on channel 3 of a 6 (5.1) channel. With PCM and specifying "5.1 channels" in output, MC successfully remaps the channels by inserting a silent channel 3. With DSD Bitstreaming, of course, DSP Studio operations are bypassed and the output from a 5 channel file, regardless of output settings, results in the SL going to the subwoofer, SR going to the SL and the SR speaker being silent. Asking MC to convert DSD to PCM fixes the channel layout.
Unfortunately, the vast majority of Classical multichannel SACD releases are in 5.0, not 5.1. For all of these, PCM conversion (or a native DSD-capable DAC) is necessary.
Considering my ancient experience with such 5.0 dscs in the past, it is a bit embarrassing that I did not recognize this sooner. Duh.