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

If it were shipped to you ,your dealer is not local.  Ship it back
as soon as it is delivered ,file a claim with your credit card company 
if the dealers refuses to give your money back ,or tell them give you a $10% refund for all your trouble. If you except a 3 rd unit . I would just ship it. Back 
call credit card company with tracking , they should give you the credit back 
after you file a dispute all the time and effort wasted . It’s totally on McIntosh.
there is no excuses  .Coda for example soak test each product48 hours then 
do a final bias and inspection before it ships ,10 year warranty parts and labor,
and the 1st year if any fault they will pay freight up and back,that is excellent service.

" McIntosh said they would replace it again but I’m frankly done, just want my money back at this point."

Why the complete turnaround?

3rd unit...

The aroma of aquatic life is strong with this one.

No refund policies are designed to discourage tire kickers, not to penalize a repeat customer with a legitimate issue/claim.  Talk to your dealer and simply tell him you have lost confidence in the manufacture to provide a reliable product.