Planars usually have rough frequency response especially in higher frequencies. Even when they are mostly flat, that curve is still rough with tens of small resonances. In time domain most planars suck big time with high frequency resonances that ring for ethernity. They are also heavy.
There was once one very special planar headphone, one that was unlike any other planars before and after. It was relatively light, just 380 grams, it had very flat frequency response apart from rolled off top end past 9 KHz. It had moderate distortion spike around 320 Hz, it was lower than Etymotic ER3/4 so it wasnt big problem.
What is most surprising was that unlike other planars ( and stats), these headphones had cleanest time domain of any full size headphone ever made. They had such amazingly fast decay and smooth frequency response like nothing else, better than HD600 or even anax modded HD800.
They were so similiar to HD600, they were like planar cousin of HD600. These headphones were discontinued, they werent liked much mostly becose they had so much high frequency roll off that they lacked too much air and their sub bass was rolled off too like HD600 albeit with superior distortion levels, they were also known for narrow soundstage like HD600.
Ultimately the lack of air, lack of bottomless sub bass extension that people expect from planar, small soundstage and boring flat tunning made then not sell well in world saturated with flavor of the month planars with fun FR and superior extension on both ends.
There was one guy who sold modded versions that were even flatter with faster decay, he fixed the treble and made the sub bass flat and extended. This was best out of the box production/mastering studio headphone ever made, better than legendary standard, the HD600. The headphone was called Oppo PM 1 and they guy who modded it was Zenith Audio.
To show you how truly special it was, here, look at these waterfall plots.
Hifiman HE5XX
Verum planar
Zenith PMx2
