Manufacturers with great customer service

The Bricasti "team"

Sean Casey (Zu Audio) and Ralph Karsten (Atma-Sphere), both very helpful and informative (patient with my questions).

Zesto customer service is top notch.  

Mondial Designs (not only replaced a problem board on used unit, but tuned & cleaned everything)

Aerial Acoustics (talked to the owner himself for information)

Odyssey Audio (replaced faulty power LED nearly instantly)

Mapleshade (nice chat on jazz after responding to question)

T+A hands down, with IAG (Quad UK) a close second.

T+A is remarkably responsive and accommodating, willing to ship replacement parts directly to customers, along with (generally unnecessary) instructions for installing them.

I interacted with Quad numerous times in the 2000s, when I was involved with restoring a # of vintage Quad valve offerings -- tuners, preamps, amps, even ESLs -- to factory spec.  Very friendly, helpful people.