Manufacturers with great customer service

Hagerman Labs. I had a hum issue while using my Cornet III phonostage. My email to customer service was the start of a 10 message thread with Jim Hagerman himself. He was very friendly and helpful. He stayed with me until we solved my problem, which was a bad tube. The unit has worked perfectly in the years since.

Pass Labs, EMM Labs/Meitner, Audio Research, Bryston. All have provided me excellent customer service for both new and used components.

Linear Tube Audio has great customer service 

KEF has been outstanding for me!

PS Audio and Pass Labs hands down