So the IMG file is not a disc image. Procedure is as follows: burn BUILD.IMG and UPDATE.VER files as data files onto a CD-R. Follow instructions for firmware upgrade. I include them as an attachment. Device is now reading SACD-Rs. It was reading burned DVD-AUDIO discs without this firmware and still does, though the particular unit I have is skipping (on DVD-As) occasionally. If the new firmware fixes this, I'll report.
After extracting the BUILD.IMG file from the SACD-R.zip archive, getting "Couldn't Mount File" and "The Disc Image File is Corrupted" type errors. This is regardless of whatever software (IMGBURN, Windows 10) image burning used. Any ideas?