Before buying the PX all my headphones were budget phones below 100£. This is my first semi expensive headphone which I bought based on the hope that will be the equivalent of the KEFLS50 wireless speakers in the headphone world. An active headphone with a quality DAC and upsampling were the sum of the parts will be greater than the parts themselves. As such it would be a really great bargain. One thing is certain for me after using them for three weeks these for sure they are not what I expected. I was fortunate enough (or maybe unfortunate since sometimes ignorance is bliss) to be given the HD 650 and oppo pm3 from a colleague from work for a home comparison. And especially for classical music and according to my taste the PX are not close enough. And although pm3 are really great I fell immediately in love with the HD 650.
I understand all the comments from all the people that find PX good enough. With House music are really great and with some type of music they provide a very solid alternative since they have attack and dynamics you can't find in hd650 or pm3. There were songs (or in movies ) that you could say I prefer them over the hd650.
But then again I understand all the people with the negative comments. There are songs or passages in music that sound, as others already describe as boxy, where PX can really go wrong according to my opinion. Most notable example is the Vivaldi piece in the 2L The Nordic sound record. According to my opinion this piece does not sound right with PX not only in comparison with Hd650 but also in comparison with the Momentum 2.0 wireless. I went yesterday at selffridges to check the gray colour and I tried again the vivaldi part to various headphones: Meze 99, Shure 1541 , momentum 2.0 wireless, Audeze Sine, Sony MX1002. For this particular piece of music I preferred all of them rather than the PX. And for me actually the hd 650 found a real competitor from the Meze 99 but this is another story...
Bottom line is that currently the PX is probably by far the best Bluetooth headphone out there but in comparison with similar priced wired headphones (and particularly for classical) is lacking behind especially in smoothness.
For the reasons mentioned above I understand all the controversy and different opinions around the sound quality of these headphones and can be attributed partially to their technological complexity. They have an amp, a DAC, they upsample everything they probably convert analogue to digital and analogue again who knows what can go wrong and were exactly. And in USB mode they sound a lot better with a jitterbug . Undoubtedly, a very fascinating headphone from a technological point of view but not exactly the panacea I was expecting.