I’m also worried about how much of the harmonics in the music is LOST by the amp?

The fine detail and harmonics is normally lost in the speaker and when you find a good one you will realize what you have been missing the whole time and keep an open mind because the good ones are not the new stuff!

But in a long chain of gear, it would be amazing to know what's getting lost where. At least with the final step, comparing speakers to headphones is one option. 

I took down the long answer. This shorter one is better. The amazing thing, what you are looking for that is lost is: music. A little or a lot is lost every inch of the way.

Everything matters. Go and listen. You will see.

speakermaster536 posts07-07-2021 The fine detail and harmonics is normally lost in the speaker and when you find a good one you will realize what you have been missing the whole time and keep an open mind because the good ones are not the new stuff!

I agree that speaker enclosures can be killers. Sound coming from the speakers is terrific, but the reverse sound trapped in the enclosure exerting pressure and interfering with the cone's free movement is something that really needs to be thought through.

I treat all internal surfaces to reduce reflections and use the best absorption filling I can find to convert sound waves to into heat. (Yeah, I know a lot of people rave about compromising the enclosure's physical volume and all that, but I've found it ain't necessarily so, or to put another way - necessarily critical.)

Modern cones and support mechanisms shouldn't be dismissed, so I'm not at all convinced "the good ones are not the new stuff!".

Reference speakers are on average starting at $15-$20 k for a decent true reference speaker on Average considering today $50k is  the mid high end range  for a speaker, there are a few exceptions to the Rule.
one key is a solid cabinet , High quality drivers , and premium Xover
components. Many companies use $30 tweeters and that’s ok for a under $5 k 
speaker ,name brand such as Accuton, Scanspeak ,Seas ,in their upper range are premium and more $$ costly but, much lower in THD . Speakers sold direct 
like Spatial Audio labs ,made in U.S.A  open baffle speaker but in the X series 
all drivers are premium especially the custom Beyma AMT tweeter- upper midband  and under $10 k a steal beating  many speakers  2x+ Their cost.