FWIW Israel did a lot of the formative work on his speaker line with a set of M-60s. We have a set of his Total Victories which the M-60s drive with ease.
But to the point of OTL vs P-P, for the most part, OTLs **are** push-pull. The elimination of the output transformer eliminates the hysteresis loss that all push-pull output transformers exhibit. Hysteresis loss is the phenomena of an output transformer where it takes a little extra energy to reverse the magnetic field in the transformer as the signal goes from positive to negative or from negative to positive.
Normally this is not a problem at moderate or higher powers, but low level detail does suffer. The reason is that the extra energy needed to change the polarity of the signal comes from the signal itself.
OTLs traditionally have more bandwidth that transformer-coupled amplifiers, and so on the right speaker (such as a Coincident) will have deeper bass with more impact, and obviously faster and more extended highs. If done right, those highs will not be brighter- just faster and more extended. There is a difference there, one that I regard as the difference between stereo and music.
There has been a lot of comment about how OTLs prefer higher impedance speakers, and while this is generally true, most OTLs of moderate power will easily handle most 8 ohm speakers. However, I should point out that while many believe that the ability to drive 4 ohms and less is a hallmark of quality, the facts of the matter are that no matter what kind of amp you have (tube, solid state, class D), if **sound quality** is your goal, then that goal and your amplifier investment dollar will be best served by a speaker that is 8 ohms or more (16 ohms is very nice), than by 4 ohms, all other things being equal. If **sound pressure** is your goal, and you have a transistor amp, then 4 ohms is preferred.
Most (not all) OTLs and tube amplifiers will make more power into higher impedances (our MA-2 and MA-3 actually make slightly less). But in most cases, driving higher impedance means (transformer-coupled or not) that distortion will be reduced and bandwidth (top and bottom) will be increased. Any time you reduce distortion, you increase detail as distortion masks detail, and the amp will sound smoother.
Except for the power issue, transistor amps will otherwise behave similarly- more detail and smoother, when driving higher impedances.
