We're sort of in violent agreement....
600ohm HP's were developed in the days when the speaker power amp outputs were being connected to HP's via a resistor network.... so High impedance meant you didn't get too much current through them - and the amps were easily capable of driving them - as with that impedance the amp can become basically a pure Voltage amp rather than needing current...
If you apply the same thing at Headphone amp level - with 600ohm HP's the circuit can focus on delivering voltage swings - and it needs to deliver substantial voltage swings (hence "hard to drive")
On the other hand opamps aren't exactly current monsters (to the best of my knowledge) - so allowing them to work primarily in the voltage domain enables them to run in their "sweet zone" and sound better.
If it's a buffered circuit, then most of the current comes from the buffer - so less of an issue (or more of an issue if it is a poor buffer)
One does wonder whether with the 600ohm HP's - the buffer circuits might become redundant as the opamps may have enough current output to drive them directly. (DEFINITELY NOT the case for 32ohm HP's where you need lots more current)
And would the sound therefore improve (due to simpler circuit) with the right opamp in a super simple high voltage output capable circuit.
So my argument is that in the world view of the 1950's through to early 1990's a 600ohm HP was considered "easy to drive" (standard HP amps were speaker amps!)
Once the Walkman took the world by storm sony and co started releasing headphones designed for small battery powered portable devices - the continued until 90% of the HP market was/is low impedance portable lightweight - low impedance.
Suddenly you have a generation of HP amps designed for low impedance headphones (that is now the default) - and with this generation of HP's classic Beyer DT880's, AGK240's and other classic headphones are considered "hard to drive"..... which is BS - they are not hard to drive, but their requirements are different - they want voltage not current.
If you try to drive them with an amp designed for the majority of the current market - then the amp struggles, and the HP's are indeed "hard to drive".
By the way if you look up the Beyer A1 amp - they provide power specs for various impedances (just like speaker amps provide power specs for 8/6/4 ohms)
In any case the Matrix M-Stage I have doesn't break a sweat with them, Neither does my Onkyo TX-SR876 receiver HP Jack, or any of my power amps through a resistor adaptor (Art Headtap).
Other devices that cope very well (superbly) with the 600ohm Revox 3100 (DT880 / 600) are my Revox Components - B225 cd player, B215 cassette deck, B261 tuner - all their HP jacks work perfectly and provide great results....
But then ALL the above are "old" tech - I haven't tried an iPod....