I’ll be first to admit that DIY is a blind spot to me, not being technical. Can a smart DIY person open up the case of say, Vladimir Lamm’s amp, and come close to duplicating the genius?
Yes but it's not easy to learn how to make PCB boards. Some parts may not be available, but hardly ever to the point to stop a copy. However sometimes the signature sound is very part dependent.
The most likely scenario is the person won't understand something so they'll replicate it wrong. Also they'll try to "improve" something but totally screw it up in the process because they never understood it to begin with. It's rare to see otherwise.
Yes, even great brands use known circuits. However the secret is in the small details. High-end audio equipmemnt is a complex system, a small difference can kill a project. Otherwise many of us would own the DartZeel chinese copies we can buy cheaply at eBay, they look really nice!
^^