All McIntosh tube power amplifiers use the Unity Coupled output circuit (a great description can be found in Bruce Rozenblit’s book “Tube Audio Design”). This inventive and effective circuit is able to run the output tubes conservatively, giving those tubes longer live so I bet the original Mac output tubes in your amp may be fine. However, the tradeoff (there’s always a tradeoff) is that the driver tubes are run hard. These are likely the 12AT7s you mentioned. I would consider replacing those first. Good news is that even though NOS 12AT7s are expensive (guitar amp aficionados gobble them up) an (almost) direct replacement tube is the 12AZ7, the only difference being the 12AZ7 pulls 400ma of filament current while the 12AT7 demands slightly less at 300ma filament current. NOS 12AZ7s are dirt cheap and common as they were mostly used in televisions, though McIntosh has used them as driver tubes in the past, such as the MC275 Gordon Gow amp, a pair of which I owned for over 20 years. During that time I only needed to replace the KT88s once, but the NOS 12AZ7 drivers needed replacement three times.
