I disagree that doubling up the current tube amp won’t gain you much. I have a pair of VAC 200iQ tube amps, and running mono is a clear and significant improvement over a single amp in stereo mode. I have 96dB / 8 ohm speakers, so the increase from 100 Watts/ch to 200 doesn’t seem like it should be meaningful. But very clearly, the pair of mono amps is much better - and it’s not always just about raw power, anyways. Also heard this same effect with the same amps in my friend’s system. And 3dB is a LOT, when you’re near your preferred listening volume in a familiar system and you’re an audiophile. "3dB isn’t much" is good forum fodder for the HT guys.
With op going from 40 to 80 watts, I’d be surprised if the improvement isn’t significant. I’d stick with the McIntosh plan.