It really comes down to efficiency:
HEK: 35 ohm, 90dB/1mW
HEX: 25 ohm, 103dB/ 1mW
HE6: 50 ohm, 83.5dB/1mW
The HE6 needs ~EIGHT times the power to play at the same volume as the HEX.
The HEK needs ~4.5 times the power of the HEX to play at the same volume.
I like translating headphone power to speaker power, just to point out how absurd overpowering CAN be (It is sometimes very much needed). Lets use the Schiit Jot, as it is cheap, a true balanced amp, and popular. With the HEX, it will be putting out over 6W into its 25 ohm load. No big deal right? Headphones are 1,000 times more efficient than speakers (Similar listening levels- Speakers at 1W, headphones 1mW). Would you put over 6,000 watts into your stereo speakers? It is FAR beyond overkill, and the likelihood of accidentally damaging the HEX increases exponentially. Balanced for portable could make great sense, but for home use I have no need.
Now the HE6 is a different animal. Right off the bat with the higher impedance, the Jot is down to 3W. Just to get the HE6 to play at the same level as the HEX at 1mW, the HE6 already needs ~240mW!!!!! Every 3dB increase from here forward for both headphones, requires a doubling of amplifier power. The HEX is still in the easy, almost free doubling area still. The HE6 is now getting into the quickly diminishing territory where power is being gobbled up.
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My goals with this hobby are to enjoy music. I rarely do critical listening, and I don't want the greatest detail because of the music I listen to (HEX is really just about as perfect as I have heard so far). We all do this for our own reasons, but once in a while it is nice to put perspective to some of it. The power race is getting insane IMO. Why buy a 10W amp if you always keep it in the lowest gain setting to drive the HEX? You paid for a lot of things in that amp that you didn't need essentially (large heavy metal heat sinks, expensive transformers, etc). Future proofing can be smart if you know you'll need it down the road, but will you REALLY need it? In general I run from the really inefficient designs (speakers and headphones). There are so many awesome sounding, efficient designs out there. I want more amp choices, not less.
I am familiar with these numbers and specs about sensitivity and impedance, and, to that extent, I am frankly not surprised that you have brought them up, but do they adequately explain why a balanced cable may or may not be required, as hifiman allegedly claims? Personally, I don't buy it, any of it, because even within Hifiman's own canon of cans, that logic does not seem to apply consistently, as demonstrated by the specs of the He-500, which I also have:
He-500 38 ohms, 89db/1mw
He-K 35 ohms, 90db/1mw
Going by these specs, the He-500 should be slightly harder to drive than the He-K, and yet it is rather my He-K which came with hifiman's stock balanced cable... The He-500 did not, which makes me suspect that they were simply trying to keep costs down, by excluding the balanced cable, as opposed to caring about that rather dubious risk of "overpowering" the He-500. I have got to say that I find this idea of "overpowering" associated with balanced cables particularly fishy, since it actually depends on how much power the user manually pumps into the cans, as opposed to the innate specs of any particular amp. Regarding the HeXV2, in particular, an example from Audeze might be interesting here
Audeze LCD-X 20 ohms, 103db/1mw
HEX: 25 ohm, 103dB/ 1mW
The Audeze LCD-X has a slightly lower impedance rating than the HeX, and the same sensitivity rating, which should make the HeX the harder to drive pair of the two (albeit by only a slight margin), yet Audeze includes a 4-pin balanced XLR cable in the the LCD-X's retail package, making it compatible with a wider, more versatile range of amps than the He-X. Does this inclusion of a balanced cable introduce any risk of overpowering the cans, a risk which will not be there without a balanced cable? Frankly, I think not... With speakers as with headphones, the risk of overdriving or overpowering resides in how one manages the power that is actually feeding the transducers, and not with the innate capacities of the amps themselves, nor with the balanced/unbalanced status of the cables.
Regarding power requirements, I have to say that I had all my high powered amps before I got either the HeX or the LCD-X, because the He-6 and the Sennheiser HD 600 and HD 800 were the cans I started needing amps for. Yet these same amps have never posed risks to the LCD-X or the HeX (when I still had it), in the same way as my 600w/8 Ohms monoblock amps have posed no risks to the 250w. 88db Tower speakers they' have been driving . I have in fact been told that, when it comes to speakers, more power is often much better than less for the quality of the listening experience, because of the headroom more power provides, allowing the speaker to "breathe" so to speak, but I digress.
When it comes to managing the power requirements of my headphones, I simply use a commonsensical loudness management approach, and everything works out nicely. In my listening practices, loudness is not a big priority... I value the richness, refinement, and nuances of the sound a lot more (which does not mean that I necessarily listen at excessively low volumes either)... But here is the kicker : with all the amps I have, and playing at the same loudness levels, using a balanced cable with the HeX produces a far richer listening experience than the stock single-ended cable ( I mean "richness" in width/depth of the soundstage, in the extension at both ends of the spectrum, the quality of the mids, in detail extraction, crispness, airiness, instrument separation etc etc). The differences were substantial enough to dissuade me from ever wanting to listen to these cans in unbalanced SE mode again, on these particular amps.
To be fair, all the amps I am referring to come equipped with both fully balanced, as well as unbalanced SE circuitries, and it is possible that this kind of topology deliberately designs the balanced circuitry to produce a sound that is superior to that of the unbalanced one. In other words, I do not know how the HeX will fare on an amp that is exclusively furnished with an unbalanced single-ended circuitry.
One last thing : I have been an active headphone enthusiast now for almost 10 years, and a speaker system enthusiast for decades... I have used the same listening and loudness/power management practices I have outlined above throughout those years for speakers and headphones, and I have never overdriven or blown any speaker or headphones... , which explains my deep skepticism regarding Hifiman's alleged claims linking balanced cables with the risk of overpowering headphones.
One more time, as we always say, YMMV.
