I suspect a lot of people wonder wonder what the difference is between QC and testing...
Testing is basically just seeing if it works after assembly. Voltages are tested and the unit is hooked up and listened to throughout the day while other stuff is being built.
You can see a video of this being done here:
https://fast.wistia.net/embed/playlists/eogduvonh0?media_0_0%5BautoPlay%5D=true&...QC consists of a lot of things. First it is visually inspected for scratches, crooked components, loose parts, the right parts, the right configuration, and then if it passes that it is checked to see if it specs out. This pretty much ensures that it is wired correctly. During this process tubes are selected for the amp and tested in the amp, until identical matched sets are achieved in both output but also in distortion signatures. The amplifier often spends several hours being cycled on and off during this process and if nothing needs to be changed a final inspection of the solder joints is done by probing each joint while it is one and watching the scope for any fluctuations. At the same time each joint is visually inspected to be sure it is a wet solder joint with a good shine on it that has penetrated the metal. Some are of course hard to see, so an endoscope is used that can reveal cracks or similar issues you can't see with the naked eye.
You can see a small part of this process, checking distortion and channel balance in the video here:
https://fast.wistia.net/embed/playlists/1rcffizxtc?media_0_0%5BautoPlay%5D=true&...If the unit gives a solid performance at the QC bench it is then listened to on a planar stereo test rig because as good as my QC equipment is, after 20 years I have learned that just because something tests perfect doesn't mean it is. The source is a battery operated iPod with 12 tracks on it that I've listened to on every component we make so it has become a great reference due to it's consistency. This set up duplicates the soundstage I would get in the listening room but in miniature right at the bench so I can immediately hear if it doesn't image right or have adequate soundstage depth.
On Anniversary gear and or mods, I usually will put it in the listening room and actually use it for a day or an evening or two just to log more hours on those tubes to catch any last minute misbehavers.