HD600 is -6dB from Harman already at 50Hz, something like -12dB at 25Hz. I'm using it with Sinxer SA-1 (unbalanced) which has enough juice but you have to turn it up considerably.
It just isn't a headphone for you if you really value deep bass. It's not easy to drive at all and with EQ, most likely needs an dedicated amp and as you said, preferably not a small mobile one. Lots of hoops to jump through if you consider other available options.
I have one and always will (like many hobbyists), but I usually have at least five other pairs to reach for at any given time, so different case than one looking for the first pair of hifi over-ears.
Harman curves are oomph oomph -voting results of/for amateurs or mobile listeners. I´m after NEUTRAL BASS in non-mobile conditions, which HD 600 pretty much achieves with that +6 dB / 25 Hz / 0.7 Q -low shelf.
Neutral bass means whole bass range on level with 1 kHz, like in speakers. I have left harman curves to harman and other bass idiots, and used this oratory1990 -graph as EQ-baseline instead
HD 600 "Optimum Hifi-curve"
Includes a little upper bass shave-down too, which affects results.
If HD 600 is not damaged and feeding amplifier up to its job, it gladly takes that much bass lift from EQ. It´s driver is good quality, and maintains linearity well (unlike some Hifi- / Hypemans etc. crap) i.e. can handle it. I have used it for years this way, and there is definitely no distortion or other problems. Just that well improved bass reproduction.
After getting its bass straight, HD 600´s divine neutrality talks itself, and makes it ready to rule again next couple of decades.
Other phones can just go to be depressed by their complete pointlessness.
You happen to be so called EIAP?
