My Amp is Broken - Not Sure What to Do - Any Recommendations?

Here’s the problem with ThermalTrak devices that supposedly was solve by Ayre but not AR https://www.stereophile.com/solidpoweramps/407ayre/index.html
ON Semi only made 1 set of thermaltraks, and yes I just confirmed that Mouser has them in stock.
By the way the SD135s are only 10 years old.

Whatever you decide, I recommend you look into a Vandy sub(or two).  Besides sounding awesome with musical bass, they will allow your speakers to be hi- passed which offloads the bass responsibility from your amp. Believe it or not, your Vandies will sound better top to bottom, not just in the bass.   The 2w’s can be found for pretty cheap

Look in the Yellow Pages and you will find dozens of competent amp techs down the street from you willing to work for $25.00/hr.  Or buy a new amp.

Distortions:  I recommend that that you formally enter a repair/service request on the ARC website.  Notwithstanding the fact that your warranty has expired, reputable equipment manufacturers will want to exhaust options available to satisfy the customer.  Since the ARC technician has already indicated there is a failure history (if not a latent defect), putting your request in text places the onus on the company to provide a formal response.  Based on my personal experience with amplifiers being blown out by lightning (electrical) and thunder (concussion) storms, I can tell you that company corporate and subordinate units really do listen and find ways to make the original purchaser happy and preserve the image of their brand.  I know it can be frustrating and time-consuming but, if necessary, you can keep going up the corporate ladder until somebody pays attention and commits to resolving the problem.  Do not simply accept the word of a first-level service technician or customer service representative, but ask to speak to their supervisor.  Best wishes for a successful outcome. 

Take ARC to task on the repair if no satisfaction  give Tom a call @ Northridge Electronics in Simi Valley Ca.   This is an old school tech who really knows his stuff and is a repairman pure and simple with no funny stuff about boutique cures or similar ilk.