I had always heard near-field as being when you were closer to the speakers than they were to each other. That is most common in desktop, but not exclusive. Optimal is usually posited to be equilateral distances. Fairfield is beyond that. But that's just my rough understanding.
On the McIntosh side, I did try all the taps. I think the issue is that the impedence curve on the NX-tremes is pretty rough, and too much for the McIntosh. It is also possible that the autoformer stuff plays worse with it than normal transformers and such in other tube amps.
I also, honestly, have trouble believing these are ever great with tubes. The sensitivity is there, but that impedence curve kinda screams "manhandle me". I say that with humility as I have not tested a lot.
All that said, I think it would like biamp or triamp really well with tubes. If you could expose each to a smaller chunk of the curve, it would be ideal.
Pre-amp - tried direct from a Chord Qutest with Roon doing fine via DSP, a miniDSP SHD doing its own volume control and an Adcom GFP-750 - none were great.
A Classe CA-201 (super old model) did better than the Mac with these two. It is demonstrably a worse amp in every way except power and damping. Thus why I strongly suspect that 'manhandle the speakers' is part of the recipe. These AVAs can do 450w into 4 ohms, so they have enough. Honestly, something high higher damping might still do even better.