Loud has good customer support. They will sell parts and give schematics to the public, not just authorized service centers.
Last time I called Hartke and needed a replacement master volume pot and replacement knob, they mailed it to me at no cost. Yes, they gave me the parts for free and they paid the shipping. Not sure why, I remember paying something last time I called them
Peavey is great as Enzo always says. they will give any schematics and sell parts to the public at a reasonalbe markup
Mesa are scoundrels IMO. They are not forthcoming with schematics and if you need a pot to replace one of those push/pull pots in Mark series they will sell it to you.............for about $15 or $20
But anyway that's not with keeping it positive.
SCIENCE amps has wonderful customer support. But maybe that's to be expected as up and comers. Pretty impressive since the owner/builder has to talk to the people too. He replies to emails faster than me and I'm sure i have way less customers.
Matamp has pretty good customer support in kind of a funny way. They always have lots of exclamation points as if they are always yelling! They usually will not give a schematic but they will offer help/info.
Soldano has amazing customer service. Soldano himself once gave me an Avenger output transformer for free because he felt bad that someone bought one of his amps and the OT failed for what seemed to be no reason. The amp was 10 years old. I had to talk him into allowing me to paypal him some money for shipping.