I haven't needed much help, but I've had really good service from McIntosh and PS Audio service.
For non-audio stuff, a big shout out to Bunn and Dyson. I had a service need from both, didn't have a purchase receipt but in both cases I gave them the serial number and they both thought I'd made the purchase within the warranty period and sent me a new Bunn coffee make and battery form a vacuum cleaner!