NAD Quality?

I have considered upgrading my vintage NAD receiver with a newer NAD unit. What concerns me is the amount of "refurbished" NAD gear that is being sold by various outlets.

Were these units returned simply because the owners were not happy with the sound, etc?

Or, were they returned because they were defective? With the amount of refurbed NAD gear that is available I have to wonder about the overall quality of the brand.