: 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
"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."
"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.