I find the APP2 are simply more detailed that the APM.
The APM sounds bigger and wider and punchier and more engulfing, but my complaint with it is that it has always lacked micro detail. It almost feels like it’s been over processed, to the point that the subtle textures and details are just smoothed away. You can hear each note and they’re nice and punchy, but then within each note, they feel like they’ve been subjected to the audio equivalent of airbrushing. In other words, you can hear a string pluck, but all those micro details and vibrations and textures within that string pluck - just feel strangely absent on the APM, so even though the headphone is clear and punchy, the over processing somehow makes it sound kind of lifeless.
Compared to most consumer headphones, the APM sound excellent. But even a 6XX kills the APM for detail retrieval.
The APP2 improves upon the APM in that regard. There’s less of that airbrushed sound, more of a dynamic range (particularly the quiet in between the notes), and I just get more texture. But then on the flip side, they sound smaller and less engulfing, probably because of the simple fact that they’re an IEM rather than an over-ear.
I find both products are well built and both are very comfortable. But I generally prefer the over-ear form factor, so I listen to the APM more when I’m at home, and I take the APP2 with me when I’m out. Then the APM are better for battery life and have a better Transparency mode, although the APP2 have a better ANC.
All in all, I think both are excellent products. But if I had to pick between them on sound quality, I’d give the edge to the APP2.