Ath M40x or Sony mdr7506

I’d encourage you to check out the Sennheiser, Beyerdynamic and AKG headphones in that price range - I think you’ll get a better experience for Pianoteq. Not a fan of the Audio-Technica headphones, don’t like the sound (personal choice). I ended up with the AKG K240 Studios at the lower end of your price range, but at around US$150 I would have gone with the Beyerdynamic DT 770.

I’ve owned Sony 7506s for years. In Pianoteq, they force me to play carefully because mistakes become very prominent - almost jarring. If I’m really trying to perfect a piece, I’ll use the 7506s. My main problem with them, they are tight on my big head and the closed back earphones with high mids really fatigue my ears after about an hour of playing.

I much preferred the AKG, Sennheiser and Beyerdynamics for everyday playing. I only play about 25% of the time with headphones so I ended up with the AKG K240 Studio semi-open back headphones (around US$70). They cause very little ear fatigue, I can wear them for 3-4 hours, no problem. If I practiced more with headphones (50% or more of the time), I would have considered the more expensive Sennheiser or Beyerdynamics, with the Beyerdynamic DT 770 being the standout for me both in sound, comfort and lack of ear fatigue.

Worth mentioning are Samson SR850 Studio Reference headphones for around US$30. These are a ridiculously great bang-for-the-buck headphones. I got a pair for my main work computer, just because they’re so great for the price. If my AKGs were lost/damaged, I could happily practice with the SR850 (they have almost the same type of design). And at 32 ohms the SR850s work well with consumer electronics - like the headphone jack of the MacBook Air. I usually have the AKG K240 Studios and Sony 7506S at around 50-70% volume. But the SR850s are comfortably loud at around 30-50% volume. One note - compared to the larger and open feel and sound of the AKG K240 Studios the SR850s sound slightly more closed, and also tighter on my head, with the Sony 7506s sounding and feeling very, very closed and tight on my head.

FWIW
- I used the MacBook Air headphone jack when I was testing headphones at different stores, so I couldn’t test headphones that require over 100+ ohms.

- In my home studio, I use the Mackie MDB-USB Stereo Direct Box (DAC). Comparing headphones on the external Mackie DAC to the MacBook Air headphone jack - I honestly cannot detect a difference in audio quality. It only makes a difference when I route the audio to large powered speakers (QSC 8.2s) then the Mackie DAC makes a difference.

I like the sound of my Grado SR80i. As always with particular headphones, you like them or not. I use them with Focusrite 2i4. Got better sound than direct from Mac mini headphone output. If you try them, don’t exclude Grado SR80 because of lack of sub bass and retro look. I chose them because of open-back and the sound. When I can here people when playing, I know the volym is not too high for me, I can hear if the phone rings if it requires my attention. Sound that leaks out don’t bother people in same room. And for me, they sound quite flat, which i need when modifying sound. The foam earpads is nice to my ears.
     And, as I have been told, they have stereo speakers and deliver sound from independent channels on both left and right sides, creating a richer sound and a better experience. If you want crisp and detailed output, these could be the one. Use good quality audio files, and they shine. It is tough to find a headphone that balances price and sound quality. These don’t blow a  hole in budget and do the job good enough.
Well, that’s what I think about it, based of course, subjectively, on my personal feelings.