My apologies_Upgrade path from Wilson WattPuppy 7's?

Regarding the degree to which the OP’s MC275 VI may be a less than optimal match for Wilson speakers, while I have no directly relevant experience I suspect the reason some consider that to be a significant issue is in most cases NOT related to the current or drive capability of the amp. Rather, I suspect that in many cases people simply prefer the sonics of other amplifiers to those of the 275, when used with Wilson speakers.

The reason I say that is that the 275 has a much lower effective output impedance than the great majority of other high quality tube amps, and hence it will come closer than most tube amps to behaving like a comparably powerful solid state amp when driving a speaker having difficult impedance characteristics, and/or impedances which vary significantly over the frequency range.

Specifically, Stereophile measured the output impedance of the MC275 V as being only 0.33 ohms from its 4 ohm tap. And I suspect that the corresponding figure for the OP’s MC275 VI is even lower, as the damping factor of the V was specified as ">14," while being ">22" for the VI. (Damping factor and output impedance are inversely proportional). Consequently, the amp will come very close to doubling power delivery into halved load impedances, as long as its maximum power capability is not exceeded.

That all seems consistent, btw, with what the OP seemed to be alluding to when he referred to the measurements his EE friend had made.

Regards,
-- Al

Ken Kantor of NHT designed Stereophile’s "simulated loudspeaker load" network for them to use for their tests, as a copy of what NHT’s speakers load an amp like.

This (the grey trace) is the McIntosh 275’s effective frequency response into that "simulated speaker load) load, (it’s acting like a tone control)
https://www.stereophile.com/images/archivesart/1010Macfig03.jpg

Now everyone here who knows their speakers, knows that the Wilson 7’s would be a far worse load than the simulated "NHT" load, have a look at any floor standing NHT load graph compared to the Wilson 7's. 
So it’s safe to say that the Mac 275’s frequency response into a "Wilson 7’s simulated load" would look even more awful than the NHT’s

Sorry Gerry, but it should be clear to most here now, that the Mac 275 is far from the ideal amp for the Wilson 7’s, unless you like listening to a tone control as an amp.

Cheers George

George, yes I saw that figure in the review of the MC275 V.

First, that figure is based on the 16 ohm tap (!), which per JA’s measurements has an output impedance of 0.87 ohms, which is well over twice as much as the 0.33 ohm output impedance of the 4 ohm tap (which is the tap that I presume would generally be used with most Wilson speakers). (Note that references to the 8 ohm tap appear identically in the captions beneath figures 1, 2, and 3, which is obviously a mistake, but the text makes clear which tap each figure applies to).

Second, the frequency response variation that is shown for the simulated speaker load, even with the 16 ohm tap, is within a range of about +0.4/-1.0 db, which doesn’t seem to me to be a big deal. Especially given that the dip of -1.0 db just occurs in a fairly narrow range of frequencies centered at a fairly high frequency, about 5 kHz. And for the 4 ohm tap the variation is within a range of an almost negligible +/- 0.25 db, as stated in the text.

Third, as I mentioned the output impedance of the MC275 VI is probably significantly less than that of the V.

Fourth, 0.33 ohms is a very small fraction of the impedances you quoted from Stereophile for the WP7:

Its impedance drops to 2.4 ohms at 78Hz and remains below 6 ohms for much of the region where music has its maximum energy (fig.1). In addition, an impedance of 4 ohms at 59Hz is associated with an electrical phase angle of -34 degrees, which will add to the drive difficulty.

Finally, as JA said in the review of the V:

... the modification of the McIntosh’s frequency response due to the interaction between this source impedance and the impedance modulus of the loudspeaker was relatively mild.

Regards,
-- Al

Ok here is the (misprint 8) 4ohm tap (gray) into the easy NHT simulated load, the Wilson will be significantly worse, probably double as bad. Especially around 100hz where the 7’s are 2ohm and -40 negative phase angle, which could equate to a EPDR of nearly 1ohm!!!! as seen by the amp.
.
If you like the sound not flat in frequency response, then good for you Al, I know you like your tubes, but the line has to be drawn somewhere between what your hear is the a flat FR, or one that’s very contoured the, 275 will be different in sound with all hard to drive speakers, such as the 7’s ect.
https://www.stereophile.com/images/archivesart/1010Macfig02.jpg

Cheers George

Here is how the Boulder amp behaves into the simulated easy NHT load, note the grey scale is almost the same as the pure resistive ones. This is how an amp should behave into the NHT’s easy simulated load.
Yes it will be a little worse into the WP7’s but nowhere near as bad as the Mac 275 would be.
https://www.stereophile.com/images/217B2150fig1.jpg

Cheers George