Really not defending Cary here. One of the few actual joys of high end luxury type products should be extraordinary support.
That being said, from there perspective: They built something years ago. It worked fine all these years and then ended up in a market it was not intended for. Somebody askes how to "modify" your unit to suit but is unable to send the actual unit for modifications.
From a customer support standpoint, Cary's biggest mistake answering your query on how to rewire at all. This is why so many companies would simply say "we don't endorse outside service" and be done with it. An unfortunate reality but you see how they get there. No good deed goes unpunished as they say...…......by trying (and failing) to be helpful they emailed themselves into a potential liability.
No excuses for them on sending the wrong advice though. THat is unforgivable but they probably though the soldering guy was more of a tech and would probe around with a meter before powering up
Also as an engineer, this is just further proof that even the most simple technical tasks require decent documentation. Casual emails don't work.
They should have sent a schematic or said no thanks.
I think you're completely misunderstanding something here.
This device was intended by the manufacturer for a conversion to 220 volts, so a transformer was installed that has 2 primary 110 volt windings. The soldering jumpers for this are already included in the layout.
This is absolutely common practice in the Hi-Fi sector.
Take a look at the documentation for the Lake People / Violectric devices.
The manufacturer could have sent documentation or referred to a download.
Printing the jumper options on the board is common practice, even for most DIY projects.
Even a sticker for 2 cents under the device or in the lid would have sufficed.
And one must not forget that this device is in the high-price segment, especially in relation to the number of components used and the simple circuitry of the power supply, tube preamp and power amp.
A DIY Whammy headphone amplifier should be similar in terms of the number of components and the complexity of the circuit.