>>106428915
Neither. HD600 and 6XX have better tuning than 660s2 and are cheaper. Arya are sibilance cannons and chinkfiman's QC is shit. If you want to get anything from them then get Sundara or HE400SE. But if you really want a planar something from DCA or Audeze would serve you better than chinkfiman. If it's just those two companies then get Sundara, HE400SE or HD6XX/HD600.
>>106428856
Planars and estats have them beat in THD and planars in particular are also easier to tune.
>>106427542
Apply a low shelf filter at 100 hz or so. A few dB would be fine though these are low distortion in the bass so even a 10 dB bass shelf would be fine.