Years ago before I got into pedal building I had a Rockbox Boiling point and wanted to increase the amount of bass in it. It already had a low-end switch. So I emailed them and they told me which caps to change out and suggested values. I thought that was pretty cool.
In contrast there is Louis Electric, who builds amps. I really like Louis Electric amps. A friend of mine had a KR12 which stopped working. I had a look and quickly worked out that it was a 5E3 circuit which used 6L6s - I think they call it a Buster. The power resistor between the reservoir cap and the second filter cap had blown and I needed to know what value it was. They paint the entire board in red paint to obscure the parts values, which is a pain and just silly. Obviously I couldn't measure it and scraping off the paint (as gently as I could) removed the stamped value info too. It was one of those white square wire-wound resistors.
So I emailed them explaining what had happened and that we were on opposite sides of the planet so sending it back for repair wasn't going to happen. They wouldn't tell me the value of the part! Stock value in a 5E3 is 5K but they had changed so much stuff in this amp I wanted to know for sure. So I used a magnifying glass, different light angles and comparison with other resistors by the same manufacturer and thought I could discern that it was a 1K. So I emailed again and they said that they wouldn't confirm that it was a 1K but that it was an excellent guess and would work.
That's just dumb.
Then there's Graydon Stuckey from GSD Amplification. He is big in the Marshall 18W amp world and has a line of chassis, transformers and amps in that niche. I recently ordered some transformers from him - Heyboers made to his spec - and they are highly regarded, rightfully so. I had emailed him asking a few details and he was very forthcoming and friendly. He asked to see pics once I'd finished the amp and was very complimentary when I did. Overall it was a great experience.