Heh heh, so true... but I was very careful in making the three setting changes to perform this SP hack, and everything works as expected, phew!
Many thanks to those who researched these mods, as I personally find the ability to record in SP and MONO with this unit to be far more valuable than the ability to (muddily) play MP3s.
I echo the warnings others have made about accessing service mode, having previously bricked a NetMD recorder by making a mistake in there (it was already dying, so no great loss). It's essential to read and understand the somewhat-confusing navigation instructions in the service manual before attempting to change any settings, and every last key-press should be made with great care!