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

I would look at the spec sheets of the new and old parts. The things to look for are maximum current, breakdown voltage, hfe. If those three things are right, then its likely no worries.

Obviously there's a little bit of engineering to do all this, but its not rocket science. Any tech (not engineer even) that actually went to school should be able to do it. Matching shouldn't be a huge deal.

If you get it right, the sound won't change. This is mostly a reliability issue. BTW $50/hour sounds cheap!

Regarding reliability of the replacement parts, ARC may have the same concerns as you; if this were my amp I would share that concern!

Be sure you don't kill yourself when you short a filter cap to the chassis (seriously).

I used to be a do-it-yourselfer, but as I got older and wiser, I realized the experience, knowledge, and training involved was not worth the expense.

Those of you who enjoy such things, by all means keep on soldering.  Most of us just want to push a button and have the stereo come on.  Asking me to match output devices is about the same as asking me to carve an amplifier out of a billet of aluminum.

"Do you guys think it would alter the sound to replace the 3281D and 1302D Thermal Trak transistors with NJL3281DG (NPN) NJL1302DG (PNP) Thermal Trak transistors, and do you think the work to match the output devices would be difficult or highly involved?"

Matching devices properly is expensive and a pain. You have to buy a ton of them to get the matched sets you need. Jon Soderberg repairs old Threshold amps, he buys transistors in lots, then sorts them according to which matches which. If you do that you’ll end up with a very large number of devices that you’ll have no use for. Unless you are going to do a lot of amps, once you get a matched set, all the rest are pretty much useless to you. Someone who does it all the time will just keep putting together matched sets. The few oddballs left will be matched up when he buys another lot.

If you have no need to replace the output devices, leave them alone. I have been fortunate enough to have always found someone who matches them, so I don’t have to. They cost a lot more, but far less than it would cost to buy enough to get a matched set!

3 matched devices per phase per channel on the SD135.
Usually you can get a matched triplet every 10 devices AT LEAST.
Now if the SD135 had say 10 devices per phase per channel that’s a completely different story.
Getting 3 devices to match is much much easier than say 5 or 6.
So figure even if BOTH channels are shot,
Buy 12-15 of each type and you can get 2 sets of each from mouser.com.
Unlike tubes, esp. those snake oil KT88 and WE300B replicas,these "sand" devices only cost a few bucks each. 2 hours of the tech’s bench time cost more than the components.
When done, sell the surplus on eBay for 1/2 the acquisition cost. heck you might even get 1 or 2 extra matched triplets good for the next hobbyist!
I am not advocating a DIY approach, just find a competent tech who knows how to match devices and bring up an  amplifier using a variac instead of a crow bar.

It depends upon how tight you want them matched I suppose, and if you want to set up a test bed for them, etc. I just have better things to do. I have a pile of gear to troubleshoot and repair as it is...