Simaudio 600i V2 volume control.

Sounds like a bad volume encoder.  Sometimes you can carefully clean the contacts.  They use a grease in some encoders that gets crusty over time and causes your exact same problem.     I have repaired several of these type of encoders.  

It could also be a faulty optical encoder but your symptoms really suggest a bad encoder.  The fact the remote works is a good indicator 

Yes, I think that it is a faulty optical encoder based on the sim system. It did work for a period last night but again today it does not. sometimes it will work other times increases volume in both directions. Very disappointed as the sound of the amp is wonderful. Time for a return. Thanks for the comments. 

Sorry to hear you need to return it .   Unfortunately that can happen when something is stored and not used for a long time.  Good luck with your search.  

That's not right.  I have the 600i v1 and am unaware of any setting that would make that possible.