If you have the capability, equipment and (importantly) the interest for measurements, plotting frequency response is one way to assess the balance. You’d find some FMs (and even sone professional reviewers) relying on measurements over trusting their ears. There’s some merit in it as our ears have their own limitations - for example our ability to hear higher frequencies reduces with age/abuse. But the counter-argument is, if you are assessing your own system (and not trying to establish its absolute virtue), why measure what you can’t/won’t hear? If you are a DIY enthusiast, its a different case as you don’t make just for yourself. But remember that measurements can only measure certain aspects (balance for example, and not timbre or immediacy), and would still have their limitations in a real listening environment (which keeps changing as you shift/add furnitures, furnishings etc.)
I don’t have the interest for measurements. Nor do I want to rely on them. So, people like me tend to rely on their ears. But then you need a reference to compare with. One option is to listen to a number of other systems and build your own internal reference. Now, that couid be error-prone. And seldom you’d get to listen to someone else’s system and your own back to back. So then what? The solution I have worked out for myself is a simpler one. Get a reference earphone/headphone. My Etymotic Research ER4SR IEM’s serve me this purpose. They are well-proven and widely acknowledged as studio reference quality. I use them as close to the source as possible. Generally I prefer connecting them to the laptop (MacBook Pro) or iPhone directly for this purpose (checking FR of my system in comparison). Now, since
@tuff visited me few weeks ago (refer above post), I’ve been experimenting with isolators, power cables and speaker position (bringing them closer to each other) and comparing the result with my reference on the Etymotics. I have been able to get some more juice into the higher frequencies (though within the natural limitations of my speakers).
In fact, I also use my iPhone (the 11 Pro Max model is quite good at it) to record samples of my system’s sound before and after any experimentation and play it through my Etymotic. The A/B comparison is very revealing and effective in assessing the balance (or tonality; though it not so for assessing say, the soundstage). In fact I can assess certain changes better this way than comparing directly (real time). Perhaps because I can focus better with the IEMs, and play the samples repeatedly.
Tuff, thanks! I love getting to know and understand personalities (also make part of my living through that

) and you are indeed an interesting person to know. Your career journey in the world of media (visualisation), your maverick choices wrt life, and your transparency as a person makes interaction with you delightful.