You might want to check with Nova Audio who I believe still licenses and manufactures Threshold product. I once needed a part a few years ago and they were very nice to deal with.
When I do a search via Bing or Google for a Threshold 400A the following link for a tech comes up that I haven't heard of when it came to repairing Threshold amps.
Ken Ealey Audio
I always thought Jon Soderberg at Vintage Amp Repair was the main man when it came to repairing Threshold amps.
Has anyone had their Threshold amps or any other amps repaired by Ken Ealey? What was his work like?
He goes into a lot of detail as to what he offers.