@fsonicsmith - I agree that phase angle is important. If the speaker's phase angle is highly capacitive at low frequencies and the impedance is also low, most techs would say that the amp should be able to produce a lot of current to get the speakers to hum. I am not sure I agree with your comment that speaker sensitivity means little. I surmise that my Ref 80S can drive my S8s with little trouble because the S8s are very sensitive (92db).
Let me challenge point you about finding an authorized ARC service center. Greg made a few suggestions to me last year. I have no reason to believe that the situation has changed. Best to call Greg.
Last, .... if you don't mind, what specifically is ailing your amp? Do you know? What is the problem?
Most of the passives (resisters, caps, etc) are low tech items. If something in the power supply regulation circuit went bad, ... that could be a different story because the Ref 150SE has a solid state power supply. It may be that a qualified tech could do the fix if the problem is not that technical. Again, ... best to call Greg.
BIF