My apologies_Upgrade path from Wilson WattPuppy 7's?

George, not that it matters all that much, but it appears the statement you quoted about the amp providing 675 WPC into 4 ohms in stereo mode (which is from a TAS review) is probably taken from marketing literature, and conflicts with the independent measurements shown at this link:

Power output (stereo mode)

• Power output at 1% THD+N: 333.3W @ 8 ohms, 499.4W @ 4 ohms
• Power output at 10% THD+N: 365.2W @ 8 ohms, 576.0W @ 4 ohm

Also, as I read its description, it does in fact use bipolar output transistors, while using JFETs and MOSFETs elsewhere in its signal path.

That said, from a technical standpoint my only reservation about this amp would be its low input impedance (spec’d at 10K/20K unbalanced/balanced, measured at 8.6K/26.6K in stereo mode, and 5.3K/15.7K in mono mode). Many tube-based components and at least a few solid state components will have problems driving those impedances, which may constrain future changes in upstream components even if it isn’t an issue now.

Gerry, best of luck with your new components.

Regards,
-- Al

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

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