Theta Dreadnaught with the McIntosh MX121?

An amplifiers compatibility with a system has more to do with your speakers than your processor. I’m positive the dreadnaught will be as good as any other amp with your MX121. The big questions are what are your speakers, how big is your room, how far back do you sit from your speakers, and how loud do you listen.

I am looking at the Double Impacts and my room is 20’ long, so versatility should not be a problem. 

Use the balanced outputs, And I don't see anything wrong with the pairing at all.
Again as said its a system match speaker match.