Two defective pieces in a row - unreasonable to demand refund?

You’re gonna have to order from someone. I would say third times a charm. I’ve been buying used Max for years and I’ve only had one problem with broken glass. I love their stuff and nothing holes or value as good as a Mcintosh. Give them a third chance see if they’ll upgrade you to even a better unit because that has such bad luck

I had McIntosh in the 70s.  It was often broken. It might have been worth it for me back then because I didn’t have a lot of options.  
Adrays wouldn’t even carry them because Mac’s QC was so poor it cost them too much.  My guy at Adrays referred to the brand as McIntrash.  It hurt my feelings but well, it’s something that you might have to resign yourself to.  Like owning a BMW - you might or might not get one that needs a lot of costly repairs - but either way you might have fun with it.  
These days I think other far less expensive stuff sounds as good or better.  YMMV.  I’m not all that experienced and perhaps that amp sounds better to you than anything else but as far as the return question goes there is no question that if you do want one they have to give it to you.  

Many years ago I was scheduled to fly into Detroit.  The day before my flight I received a call from a teammate asking if I had seen the news.  It turns out the flight I was scheduled for the next day crashed.  When I said I would see them the next day they asked if was nuts... to which I replied,  what were the odds of the same flight crashing two days in a row.

Anyway, what are the odds of having another issue with the 3rd unit?  I'd be chill and let the dealer and Mac get you sorted out.  

I would not unpack it at the dealer unless they were willing to repack it.  I might ask them to deliver it and make sure it works at your house.

I don't know why this thread goes on and on with suggestions. OP already said long ago that the is ordering a third unit. Enough already.

Update:

Finally up and running with a spotless and functional unit. Chalk my problems up to bad luck, or COVID, or whatever. McIntosh and dealer took care of me with no hassle. Easy to forgive problems when the manufacturer (and dealer) makes it right without skipping a beat. No regrets, sounds amazing. Bottom line, I would advise anyone who reads this thread in the future not to be scared off McIntosh because my experience. It is true that I had to go through multiple units till they got it right, but they got it right and didn't give me any grief, which counts for a lot in my book.