On the speaker impedance: Speakers are horribly complex loads. Unfortunately, most amps are tested with resistive loads. Flat response into 8 Ohms etc. Well speakers have resonance peaks, phase shifts, dips or peaks due to the crossover and rising with frequency. Typically. Good designers can minimize these to some extent. Now a well designed amplifier is a VOLTAGE amplifier and should care not what the load does. It should supply whatever current is needed for the voltage gain. 2 to 30 Ohms from 1/4W to full power, 20 to 20K is a fair range. One clue is if the power @ 4W is double that of 8 and they claim 2 Ohm stable. Usually this is a function of power supply and feedback implementation. Poor class D amps seem to have great difficulty with this. Newer ones, just fine. ( I still don't seem to like them though). Solid State can do this. Tubes are tougher as the transformer would like to see a narrower impedance range as a load. Probably why synergy with tubes is a litter harder than with SS. This issue is not a mystery and can be measured but it is not a single number to be advertised.
Mastering is a problem. Billy Joel recordings seem to have 4100 really boosted. Loudness wars peaking to 0 dB which really screws up digital filters. Moody Blues were just plain terrible. Early Beatles really hard Left and Right, not stereo. Sheffield mastered very low level for wider dynamic range. The only thing I can think of is to evaluate and pre-process every track in the digital domain and store the processed results for playback.
Trying to get used to my new Schiit Vidar. Quite clean. Not quite Benchmark, but clean. Different from my MOSFET ( my own). Seems to do drum impacts cleaner but less dynamic. Better control so keeps the drivers from flopping in the breeze? Am I used to a sloppy sound? I converted my mains to sealed from ported at the same time, so a difference there. ( I prefer low Q sealed alignments) My main focus is on female vocals and possible sibilance, then if solo instruments sound right. Guitar, obo, flute. I gave up on pianos as I don't think one has ever been recorded well.
Listened to a lot of speakers recently up to about 5K. Of all of them I find I liked the Sonas Faber Lumina''s the best but at medium volume. They can't really do a Beethoven crescendo. Revels were very well balanced, but just can't take the hard dome. Not much else I liked at all. I tried a couple hard domes in my speakers (Seas ER18 paper woofers, and a series of domes and crossovers) Back to the SB soft dome. I was hoping the AMT's would advance now the patents expired, but nope, every one I heard is terrible. Air with a big helping of distortion to go with it.
The end result is that is is still up to the individual. Hearing is our brains interpretation of the sound. It is not real. We all have our sensitivities and preferences so "right" is only in the eyes ( ears) or the individual. I can't tell you what you will hear. I may know a power cord can't by the laws of physics make a difference in "air" , but that does not mean people who believe they do are making it up when they hear it. They hear it, real or not. Our brains are the biggest liars on the planet.
PS: I have heard soft domes break up and sizzle worse than a $2 aluminum one. So, like everything else, "it depends" I just have not heard a metal dome I liked including the new Be uber-priced ones. I would say it is the tweeter, not the crossover as I have tried to tame several SEAS metal domes, got fine free-field response, but not liked them.