Great amp the S500, bought mine blown cheap, and repaired it, loves the bias set so the heatsinks run at 60c. I’m in Australia so I can’t help you.
No need for a Threshold specific tech, just a good solid state tech would do as they’re quite easy to work on, as nearly all the circuits are on the net and good techs know how to search them out.
To find good techs ring around hifi shops that carry some hiend gear, and see who they use, as most now use outside repair companies.
Cheers George