Volume Adjustment

It may be possible there is an output impedance interaction anomaly between your MA-252 and the speakers. Neither is at fault, it's just caused by how they happen to interact at that point. 
Have you tried hooking up some different speakers to see if the effect happens with different speakers in the same way at the same setting? If it doesn't then you probably have an output impedance anomaly between the amp and those speakers. Kind of a flukey thing but it could be the cause.

Would be nice to have volume adjustment for one’s spouse….jokes like this can land me in the doghouse.  Perhaps I should expand the doghouse and add a few amenities…like a stereo system…

It is probably not linear, but some form of logarithmic response. Create a chart of output vs setting number. Plot it. The chart will show if there's a jump or a break from a smooth curve. Curve should be smooth. Not necessarily linear, but smooth without big jumps.