Yes, Charles; it appears that you do have several misconceptions with respect to amplification, particularly as to audibility of distortion and what clipping is. There are no "undistorted watts", the question is how many aren't audibly distorted. In listening to music, tests have shown that distortion has to be more than about 1% to have audible effect. For example, in
FAQs Yamaha applies a "less than 2%" standard. Arguments we've seen on other boards anguishing over whether a .05% THD rated receiver sounds better than one rated at .09% lack any sense of reality.
Clipping is distinct from distortion, although at actual clipping distortion would be quite high, at least several percent(without defining "several" too closely, since it can vary). It therefore occurs well after the onset of audible distortion and is evidenced on an oscilloscope by the flattening of the top of the sine wave. Magazines which use the term clipping at 1% or even lower, when apparently they actually mean distortion at an inaudible level, are a source of confusion if correct audio terminology is to be employed.
The calculations done for Dave were simple and quick. The anechoic sensitivity of the M80s(room contribution is accounted for differently here)is given by Axiom as 91dB, while the NRC, apparently measuring a different range or other factor, gives 89.5dB. Using 90dB at one meter the typical in-room results for home listening rooms would decline at about 3dB per doubling of distance, taking into account both direct sound and room contribution. This has been researched and Dr.Toole discusses this in his book. Therefore at the 12' distance Dave specified there would be about a 5dB drop from the one meter measure, to about 85dB for 1 watt. Hence the supplied numbers of about a tenth of a watt for the specified 75dB level and 100 times higher to allow for a 20dB peak on the most dynamic of material,(such as some of the classical recordings which I have)to about 10 watts. Also the follow-up number of 104-105dB for a peak using about 90 watts is similarly calculated. These levels are of course those for long-term home listening at home with a view to avoid permanent hearing loss, not those occurring at the recording site.
The linked lab tests on the 1909 confirm its excellence, including the 4ohm measurements. The comments of the reviewer on this point aren't explained and appear to be somewhat nonsensical in that they directly contradict the measured results. If the 140 watt number at 4ohms(certainly not actually "clipping")is used instead of 90 watts, an extra 2dB could be added to the calculation, i.e., now 106-107dB.
