Of course aftermarket transistors will work. Transistors are color coded so they can be identified. Take close up pics of them, look up the color codes and then call a couple of parts houses and get the tight tolerance versions of them and have your tech install them.
McIntosh C28 transistor subs
My C28 needs two transistors that are on backorder from McIntosh for months now. Can't a high quality brand transistor with similar (or better) specs do the trick? I do not know the part number of the transistors in question, but my repairman wants to await the arrival of the McIntosh-supplied parts. Your input is welcomed!