Mx135 subwoofer not working???

I owned an MX135 but it's been a few years, so forgive my "guess
from faded memory". But I seem to recall that you must set the cross
over in the 2 channel settings menu, but I really seem to recall that the
"pure" excludes the subwoofer. Also if you haven't experienced
this, I definitely do remember having to do a capacitance reset, by
unplugging the unit and press every switch and turn the knobs while it is
unplugged. That allows the processors to return to their default settings,
and sometimes that helps get a setting change to be accepted.
Good luck, but look at your manual, I really do think that the subwoofer is
not used in pure mode. But could be wrong, dang that could hurt! :)

Tom, it does sound heat related. I would call McIntosh, they are very good at helping you diagnose and also in the past they done repairs for me, and although out of warranty, it was still free. I just paid the shipping to them and they sent it back at no charge. Not saying it is policy, but worth a try.
They never indicated it would be free before I sent it, it just worked out that way. They are a great company to work with.