Volume Adjustment

From your description it seems that there is something amiss with the volume control. There should not be such a big difference with a 1 decibel change!

something is wrong. measure with an spl meter to document the change, then give McIntosh the facts.

you can buy an inexpensive one (with a bottom hole for a tripod), it's good to have one

https://www.amazon.com/RISEPRO-Decibel-Meter-Digital-Sound/dp/B01EZZ8B5Q/ref=sr_1_15?sr=8-15

@teboerio  

The pot may have oxidized. DeOxit spray can be used, but be careful to keep it away from other components. Let it dry thoroughly before turning power on again.

If this doesn't work, it may be possible to disassemble the pot and clean inside it.

That is probably a job for your tech.

That sounds like an issue with the rheostat. I would talk to McIntosh. 

@kevemaher 

I would avoid using DeOxit for the time being if I were him.

If there is a hash-like sound or a crackling sound when you turn the volume up or down, it usually means the attenuator/knobs need to be cleaned, as they are making contact but struggling to do so consistently. In that case, DeOxit is the first thing you use.