OK, I finally had a bit more time to investigate the silent R surround channel, and I think I finally figured out what is happening. It is not so much a bug in JRiver but a awkward feature that makes it too easy to enter wrong settings. I am surprised that no one else playing multichannel SACD files has come across this issue, or else has not registered that one of the surrounds is silent.
Here are the facts:
1. Some of the mch SACD tracks are 6-channel (5.1 configuration), while others (mostly some classical labels) are 5-channel (5.0 with no discrete LFE channel). JRiver "detailed view" correctly reports which ones are which
2. The way channels are given numerical assignments in JRiver are 1= front left; 2 = front right, ...4 = subwoofer, ..., 6 = surround right
3. In the DSP studio window, I had the # of channels set to "source # of channels" (Which I thought was reasonable).
4. All channels played when the mch SACD track was of the 5.1 variety, and the R surround channel was quiet when it was the 5.0 variety
My conclusion is that the mch signal from 5.0 tracks was truncated at 5 channels only, leaving out the last sixth (R surround) channel. So, I changed the Channels setting from "source #" to "5.1" and all became well. In retrospect, maybe it was to obvious of an error, so no one but me made it.