To monoblock or not to monoblock McIntosh MC275's

^^^^^^^^ Al, thanks so much for this clear explanation. Of course, I expect nothing less from you! But, neither do I take it for granted.

It does sound like the two channels of a stereo amplifier must be very closely matched for parallel operation to work well. Because we’re talking about tube amps, I wonder how practical that really is.

What's really odd about this is that, many moons ago, I had a Dynaco ST-70, but never knew it could do this. It's always nice to learn something new.

Thanks very much for the nice words, Cleeds. Regarding ...

It does sound like the two channels of a stereo amplifier must be very closely matched for parallel operation to work well. Because we’re talking about tube amps, I wonder how practical that really is.

... I suspect that in the particular case of the MC275 a factor that helps it avoid or minimize any sonic degradation when operated in parallel mono mode is that like many McIntosh amps it apparently uses a substantial amount of feedback. One indication of its liberal use of feedback being its specified damping factor (">22" for the Mk VI version), which is quite high for a tube amp.

In general, feedback will reduce the degree to which the behavior of an amplification channel is affected by variations in tube parameters, or variations in other circuit elements for that matter.

Best regards,
-- Al

@almarg :  Thanks for your input; always expert and most welcome.  For reference on SPL sensitivity, you may wish to see http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/db.html#c4 where

"JND in Sound Intensity

A useful general reference is that the just noticeable difference in sound intensity for the human ear is about 1 decibel.

JND = 1 decibel
In fact, the use of the factor of 10 in the definition of the decibel is to create a unit which is about the least detectable change in sound intensity.

That having been established, it can be noted that there are some variations. The jnd is about 1 dB for soft sounds around 30-40 dB at low and midrange freqencies. It may drop to 1/3 to 1/2 a decibel for loud sounds.

Caution must be used in applying the "one decibel" criterion. It presumes that you are increasing the same sound by one decibel. If you were adding a sound outside the critical band of frequency from this sound, you would be exciting fresh nerve endings, and the one decibel rule can't be presumed to apply. This causes some concern about the perceptual encoding schemes used with modern digital recording which might eliminate some significant audible content by the use of a "one decibel" criterion for dropping content."

And http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/db.html#c5, where 

"Variations in Difference Threshold

[Note:  graph can't be pasted here; see link]

The above data are from Backus, suggesting that the JND in dB is less for more intense sounds. He is citing Harvey Fletcher's "Speech and Hearing in Communication"(1953),p146, as the actual data source. But you can do a test for yourself of pairs of tones that are stated to be 2dB different at the McGraw-Hill site. This site discusses "Weber's law", which states just the opposite of the implication of the above curves."

Academia hasn't reached a consensus on the point and I haven't tested the McGraw-Hill applet yet myself to see where I might fall on the scale.  Since music has far more tonal complexity than the scientific examples referenced, it's difficult to state what true JND might be for a given source material.  1 dB is correct for a single tone in a comparatively quiet environment and may well not be under other circumstances.

I can't recall now where the 3 dB value I noted above was cited, but I believe it may well have been based on Fechner's "Elements of Psychophysics" (quick reference:  https://en.wikipedia.org/wiki/Absolute_threshold_of_hearing) and the application of individual subject testing statistical analyses to it.  Possibly a mean or median with a tight standard deviation?  My coursework on it all was a long time ago.  It sure was bandied about with confidence at the audio store I worked at (Accoustat, Ariston, DCM, Dynavector, Hafler, H-K, RGR, Rogers, SME were the higher end lines during my time there).

Regardless and getting back to the matter at hand, a pair of MC275s in mono will certainly improve image and definition notably.  SPL output perhaps not so much.  And as you go on to note, any 2 amps rigged for mono would have to be similar enough in amplification characteristics to be transparent L to R, which is less of an issue with the 275 (due to feedback) provided the tubes are relatively uniform in performance across both units.  Stereophile's review of the V (Kaplan) notes that there is only one difference between it and the IV (reviewed by Tellig):  Binding posts versus terminals.  Depending on the spade size selected, that might or might not be an issue.  Either way, it's not an issue for the OP as he's already sold his IV.  

I still feel the OP could give two 275s a shot if he likes the sound of the 275.  It's a nice amp regardless of generation and since he has the luxury of being able to wait until a good value on a series V pops up, two could be just the ticket for him.  That's the fun part of this hobby - fooling around with configurations for happy listening!

Thanks again for the expert and valuable input.

Yeah, I should have tried it on my system, and I was a bit silly not to even open the box because I wanted to keep its value. It's a McIntosh. It doesn't matter!
Luck has it that I may be able to get another MK IV and put an end to this and see for myself if it makes a difference. 
My XRT 28's are rate 1200 watts and sound great and pretty loud with only 75 WPC from a single MC275.
I'll post my impression if I get to test another MK IV. (or 5)

Effischer, thanks very much for the nice words, informative references, and good comments.  Interestingly, I took the "Weber's Law" test four times, using four different settings of the volume control on the computer.  On the low volume trials within each test my answers were MUCH better (averaging 85% correct) than on the high volume trials (averaging 30% correct).  So between the two conflicting references that are cited at the second link you provided, my results would indicate that the Weber's Law reference is the correct one.

Thanks again.  Best regards,
-- Al