Nah, don't worry about specs at all until you hear the device. Plug in the headphones and listen to them, if you like how they sound, specs are irrelevant. I went on a little quest earlier this years trying out as many amps and receivers as I could to test out their headphone outputs, since almost everyone was claiming their headphone outputs are bad and are only an afterthought. Well, WRONG.
I ended up listening to about 20 of them in the <500$ range, from Yamaha, Denon, Marantz, Onkyo, HK, Nad, Pioneer, etc. ALL had excellent headphone outputs that sounded great with high and low impedance headphones. Amps sounded slightly better than receivers (Onkyo, Denon and Marantz had the best sounding headphone outputs), and they pretty much all sounded as good or better than the dedicated DAC/head amps that I had tested at the time as well, Asus Essence One, Teac UD-H01 and CA Dacmagic+.
At the end I ended up buying an Onkyo A-5VL amp with an integrated DAC without knowing a single piece of spec about it. I bought the thing purely on listening experience, haven't heard of it before I entered the store, never read a single review on it. And that's how everyone should buy their gear, don't read reviews, try it, compare it in the store, don't believe anyone on the internet, especially not official magazines, you never know how much they get paid when they get a review unit. Don't even believe me now, go into a store and try it for yourself.
All receivers / amps I tested had plenty of power for any headphones I tried them with (DT880 600 ohm, Q701 and HD650, so all impedances covered). What all receivers and amps had in common in terms of sound was musicality, punch and size. I don't know how to explain it, but all headphones sounded "bigger", I guess the soundstage was bigger, bass was punchier and more voluminous (probably due to lower damping factor) and music just sounded nicer, somehow more coherent and just more involving. I guess that's what you get when you buy a real properly engineered audio component, instead of paying 500$ for a small aluminum case the size of your palm with less than 100$ worth of components inside, which lets face it, is what you get with most dedicated head amps these days. After getting this 22 pound heavy amp which has so many inputs, sounds excellent with speakers and headphones, is built like a tank, looks and feels expensive, has a top of the line Burr Brown 24/192 DAC inside, bi-wiring possibility, etc. and all that for 400$, I just cant justify buying a head amp or dedicated DAC. I look at my Little Dot MkV and Onkyo A-5VL, and I think to myself, how is it legal that these two things cost the same?
My favorite receivers were:
Onkyo NR509
Marantz NR1602
And integrated amps:
Onkyo A-5VL (which I ended up buying as I felt it was best overall in terms of sound/cost/features)
Marantz PM5004
Denon PMA720AE