bflat
Headphoneus Supremus
I wonder if, over time, software gets slightly corrupted and installing new software just cleans things up. I believe A&K when it says, no change. So either it's phantom changes or clean install just makes a slight difference.
If a firmware update changes the operations of the SOC, theoretically the sound may change. This is based on the my understanding that SOC chips tend to generate quite a bit of noise. Rob Watts from Chord had a lot to say about this in another thread. If one were to stretch this theory further, the SS and CU chassis may have different characteristics in dealing with EMI/RFI coming off the SOC chips which may also alter sound.
I'm no expert, but I suspect if any of the above is true, the sound differences would be really subtle. Probably grasping for answers though