Just a thought but did the 18bit versions have flashable FW? If you can flash them that may be the answer? If you press display on the MSR/MSR+ whilst the speakers are in standby the speakers will display the current installed FW.

Is there any chance you mixed up the rear pair and your spare set at some stage?

All the best

Richard

Main system; 861v8 ID41, DSP8000SEU fronts with DSP5500 centre and DSP7000 rears (all in lovely silver) 504, HD621
Second system 861v4 ID40 LPS, G98DH, 504, HD621, 2x DSP5200 with DSP5200 centre
MD600 tucked away in study. Lots of other Meridian!