By the way, as I was reading up on the various open back Sennheiser models, I came across a helpful graph that helps talk about response:
A lot of these descriptions make sense to me, and match what I’ve heard dragging a notch filter around in an equalizer while mixing audio. Anyway, just leaving this here mostly so I know where to find it. 
Side note: for those considering Sennheiser HD58X or HD6XX headphones, going straight to drop.com gives you (atm) better prices than on Amazon or eBay, and on drop they’ll be guaranteed brand new.
I’ve got an HD6XX pair on order. Very different product than the APM (open back, obviously no active noise cancellation), but hopefully better sound. Perhaps I should have gotten the HD58X: the 6XX are supposedly way better, but at 300 ohms (vs. 150 of the 58X) they might be too much for my Fiio BTR5. Edit: it’s fine. BTR5 at max volume and iPhone at 50% is plenty loud. I did order a $15 balanced cable so the BTR5 can use its two DACs for more volume/less risk that the lower end output is limited by the preamp.
So why the 6xx? The 58X has smaller drivers, generally considered lower-end than the 600 series. I want something “impressively better” than what I own, so cheapening out makes no sense. Between the 600, 650, and 6XX, it seems that the consensus is that the 600 is more neutral/reference, with a bit less bass, and that the 650 and 6XX are virtually identical, minus some of the accessories included. The 6XX is way cheaper atm, I paid $233 including tax and shipping, compared to $400 (“from $550”) for the HD650 before tax from Sennheiser. FWIW, the 6xx were down to as low as $180 around Christmas time last year.
Anyway, even if this purchase ends up causing me to buy a new preamp, it’s probably still cheaper than my AirPods Max spur of the moment purchase. 
And hopefully I’ll enjoy the sound a lot more!