Volume Adjustment

Also w test tones and a decent SPL meter you are ahead of many here….. 

The McIntosh MA252 does not use an old-style rheostat for volume control. Instead, its electronic attenuation system (reflected in the volume display) follows a logarithmic attenuation curve. As a result, the volume increase per step is not uniform; in some regions, each step may represent a larger change than in others.

It is quite possible that the 52–53 transition crosses one of these regions where the attenuation curve changes slope. My Cambridge Audio Azur 851A integrated amplifier has a similar type of volume control behavior, and I can also notice a slight jump at certain points on the dial.

On my system, the jump is noticeable but still within expectations. If you feel the step change is unusually large compared to normal behavior, it may be worth raising the question with McIntosh for confirmation.

I had an ARC preamp that had a blown shift register and it created what you are describing

I have an MA252 and have noticed no such phenomenon anywhere between volume settings of 30 and 85 (loudest I can set it before Amber lights indicate clipping levels). may be your device needs service. 

How are we supposed to know what an MA_52 is?  
This is a pet peeve of mine.  Why can’t people thoroughly describe their equipment instead of assuming we know what they are talking about.