You're welcome!
It depends on the source material and one's HF hearing response. At best, for men AFAIK, they will perceive a bit more detail/'air' around some instruments while for at least some women it's the difference between a properly tonally balanced HF harmonic structure or not. Indeed, I found that for noisy and/or sloppy recordings, at least younger women preferred a system that was pretty much done by 15 kHz, especially if listening to FM radio.
On axis and depending on where the HF adjustment is set and assuming a fresh diaphragm, the M-19 is solid to 20 kHz.
Regardless, adding a super tweeter isn't so much about getting to 20 kHz than it is about the sound quality (SQ) of the HF and controlling it over a wider 'sweet spot'. Even the 1" exit drivers are just making break-up noise off its suspension at these high frequencies with it's throat size controlling them, ergo some reflections get added to the mix since it's much too big for 20 kHz and if the mouth isn't damped enough, then between more reflective 'scatter' off it and eigenmodes reflecting back to the throat to modulate the delayed HF, there's no real SQ.
A larger diaphragm/throat then will start breaking up sooner and add more reflections to the mix, so HF SQ can only be worse, ergo EQing it flat out to 15-20 kHz isn't something I'm going to recommend unless it's coupled to a plenum loaded mega multi-cell similar to Jensen's 32 cell behemoth where there's so many close coupled super tweeter horns that we perceive it as a huge, very wide BW point source.
Anyway, as 'bowtie' noted in your other thread, there's duplex compression drivers available now to deal with this once frustrating 'juggling act' of trade-offs.
GM
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