There may be minor production line changes along the way, but if there are major changes the model number will be modified. The model number might be completely different or a simple addition such as a “Series 2”. There have been no significant changes to ARC.
“Refurbished” units have been returned for some reason. Units purchased directly from SONOS have been verified to be in working condition -- but there could be minor cosmetic imperfections. Reasons for return vary. Perhaps the unit does not sound as expected or a significant other declared the unit as “ugly”. In some cases the customer purchased the wrong unit -- such as purchasing a PORT when they should have purchased AMP.