[Ringmaster]

After two years, my NADs decided to call it quits; the left speaker no longer works, no matter what cable I use. I'm looking for a similar pair of headphones as I loved the sound of the NADs, but now I want a more robust build quality. The Oppo PM3s appear to have an excellent build quality, but how do they compare to the NADs soundwise? And I'm not set on the PM3s either; I wouldn't mind a complete upgrade so as long as it stays under $1k. 
 
What are your recommendations? Portability, decent isolation and little noise leak are also a must. I'm currently using an iPhone 6S/Mac and an Audioquest Dragonfly DAC/amp to drive my headphones, but I wouldn't mind upgrading if a particular pair of headphones called for a better amp. (I do like the convenience of headphones that sound decent unamped, however.)

 

[Beyakusenn]

The PM-3 is one of the best truly portable headphones, but there is a new planar magnetic portable headphone that does very well with Apple devices: the Audeze Sine. It has a lightning connector with an in-line DAC/amp that is made specifically for the Sine. If you want to listen with other devices, you can just use the cable with the 3.5 mm mini-jack.
I haven't heard the sine myself and I do not have Apple products, but I think that for you it's worth looking into.

 

[Bart147]

Both PSB M4U 1 & M4U 2 have a similar "fun" sound signature as the NAD HP50 and won't drain your wallet as much as the PM-3 .
The PM-3 is more analytical and detail oriented which delivers a completely different listening experience , if at all possible : let your ears decide in a side-by-side comparison like I did at the headroom exhibtion in London .
You will like the Audeze Sine if you can live with an on ear headphone , I found them uncomfortable after ten minutes of listening.