Lordy, this is quite a mess.
A quick Google turns up a few internals pics here:
https://www.superbestaudiofriends.o...ve-volume-control-for-dacs-sources.860/page-9
Forum comes compete with a snide remark about Amir.
Post above has the other pic I found.
The preamp looks like a total amateur build. There is so much wrong with it I would be utterly ashamed to put my name anywhere near something like this product, and as to charging $1k for it, it is just ridiculous.
The switch on the back is a chassis ground lift. Really. And the switch would never survive a realistic fault current. It might weld itself solid, it might vaporise before a breaker blew leaving you with a lethal live chassis. Some versions don't have the lift switch. It seems then they just don't bother to ground the chassis. The mains wiring inside is not to standard either.
The quality of the soldering is beyond poor. It is what I might expect of a 12 year old doing a circuit build for the first time with poor quality tools. The entire internal layout and construction is quite simply terrible and not of merchantable quality. There is clearly no understanding of ground loops and management of ground return, which will be a contributor to the mains pickup and poor noise figure. Not a cable tie in sight, which along with the terrible soldering will lead to poor reliability.
Now to the circuit. It is an op-amp buffer on the input and output coupled by a light dependant resistor lit by a LED controlled by the volume pot. That is it. There is no light shielding for the LDRs so the chassis had better be light tight to avoid interference from external light sources - especially modern LED lights flickering away. This is a funny idea that keeps raising its head from time to time. The volume control on the front doesn't touch the signal, but just runs the LED. But there are all manner of issues with the idea, poor noise being one of them, and unclear distortion effects in the resistor.
This is a product that needs calling out for what it is. No matter what else, its construction leaves it as an unsafe product that should not be sold.
There needs to be a panther past the headless one. This is deserving of it.