This took a while but... My view is that it's on the same sort of level as PC hardware having been in that game for literal decades. For example the latest gaming mice in the top tier groups costing £200+, do they offer better sensor tracking than one costing half the price? Nope, they may even be using the exact same sensor. It's the subtle nuances you're paying extra for, or an overall package of refinements over the cheaper one such as build quality or ergonomics or weight. I realised this fairly quickly as a tester of both entry level and high end mice and keyboards over the last year or so.
In relation to audio, not so different, things change as the industry moves on with the tech, what was once expensive is now pittance and can be featured on everything, and yeah the marketing departments for many of these brands may go OTT with how they promote certain things, but those who know will just gloss over them and make use of the feature in whatever use-case they may have for it, balanced HP output being twice as powerful as unbalanced, for example, so driving those hungry headphones is much easier without having to max out the volume dial.
On the volume (power) front, I found out that you don't need much power to drive most headphones to deafening levels of loudness. My first third party DAC/Amp once I finally sacked off NAD and their planned obsolescence just out of warranty issues, was the Topping MX3S, that amp has a headphones output of only 100mW at 300 ohms, and the HD650 more than loud enough at moderate volume at high gain on that. no problems whatsoever, no distortion or anything.
Once I started getting interested in Planars it became obvious that the MX3S didn't have circuitry even with its 700mW at 32 ohms to drive massive magnet arrays and would instead trip the protection mode, that's where the search began for a better suited headphones amp for that purpose.
In that journey I realised that the point of diminishing returns is now priced so low that you can't go wrong with even entry level DACs/amps or dongle DACs even. What matters more is the quality of the headphones or speakers. The more expensive DACs/amps in the chain offer subtle nuances here and there, bit of refined soundstage here, expanded stereo imaging there, both of which are subjective anyway, though Planars being bright in nature would indicate a smoother and warmer amp being best suited to them, which just so happened to be my exact sound signature preference, which I often call the "NAD sound".
As you said, no reason to not go balanced now, otherwise you're only making use of half the amp, the cables are cheap, the headphones all support it, the amps are engineered for it (as long as fully balanced....).
In the simplest form, is a £1000 DAC/amp £800 better sounding than one that cost £200? Nope, the point of diminishing returns note from earlier comes back into discussion.
Likewise was the more powerful Topping DX3 Pro+ better than the MX3S for headphones? Nope, in fact it had features cut, a worse volume pot and died after a week or so of use, amazing!
Same question could be asked about the HD660S2 vs the HD650, I had the 660S2 and did not think they sounded twice as good as the HD650 given they cost double the price. A bit more sub-bass and more veil and that was it. I had the FiiO K7 around that time so plenty of power as well for the sake of scaling.
With the X9 DAC, I just found the higher end unit offers is considerably better build quality, internal component design being better thought out such as high quality PCB, thick traces, isolated power boards and noise control, connecting cable quality between jacks and boards, usability ergonomics and some USP features not found anywhere else.
The sound is still amazing through them all, but there are obvious refinements here and there again in stuff like soundstage and imaging, but the plus point now is that on units like the X9, you can tailor those refinements however you like thanks to those USP features. I don't personally use them as bypass mode offers enough refinement over what I've tried before with my specific speakers and headphones that I'd prefer to keep the source input signal as honest as possible but just have those subtle refinements in place and this then becomes my new reference setup to compare other stuff against when stuff gets sent in - And since it cost me nothing, I've not had to wedge out extra cash for what would otherwise be subtle improvements. But given those features are now there, I know that I can make use of them on other headphones down the line which adds service longevity to such a product.
What all these paragraphs of text mean is essentially that it's all hugely subjective and specifically boils down to individual use-cases and the type of headphones you have vs the type of sound signature you prefer. Me I don't care about the most precise resolution possible, to me that sounds too clinical/sterile and makes music boring, even if it is very detailed. I want music to sound fun, and sometimes holding back some of that high range detail but bumping the soundstage and imaging is the way to go, at least with Hifiman {lanars. Will that change when the LCD-5 and Fosi i5 arrive? I don't think so as I'm quite specific with the kind of sound i like, but reading comments and other reviews of the lCD-5 has given me the impression that they are very clinical as they focus on resolving power over making music "fun".
Though then again I have a smooth and warm sounding amp with the ability to completely change the signature of any headphone with HP-EQ, so it will be interesting how that plays out with the LCD-5.