Try the audiokarma forum, they have threads dedicated to McIntosh and technician issues like this.
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!