Received the Custom One Pro Plus from Beyerdynamic today. First impressions is that, I absolutely lovee their headband (holy comfy cushion Batman!). SO much better than the headbands used on either Pro/Premium DT770/880/990/T70 lines.
Haven't had much real time with them other than nothing how comfy they are (still wish they had included DT770 velours in the package, though the pleather is super soft and comfy).
The included audio cable seems to be defective (cuts off on the left channel with both my phone and my E17). It does come with a cable with an inline mic, but I'm just using the custom cable Kamakahah gave me for the X1 ages ago. Regular cables will fit the COP = V-moda Boompro will work).
I haven't listened to it much but first impressions:
Warm, clear mid range (OMG upfront midrange from Beyer!), smooth treble
Bass sliders are completely functional, and not as overwhelming as I believed. Bass slider on 1 (no ports open) = zero bass. I don't think ANYONE will use this. Bass slider 2 and 3 are very, very similar, and how I feel everyone will use the COP. I'll need more time to see just how different they are, though it's definitely subtle. Bass slider 4, I expected overwhelming bass, but I don't hear all that much of a boost compared to 3. I'll need more testing, but it does sound a bit sloppier, not overly emphasized.
What I like is that the bass sliders seem to affect sub bass more than mid bass, which makes me happy, as I'm a sub bass kind of guy, and don't mind a reigned in mid bass.
The COP seems to be good for long period use. No real ear fatigue, in terms of it's tonality.
Super early verdict: I like their sound and comfort. It doesn't seem to be as energetic as the M50 or CAL in terms of splashy treble, so while the other two may sound more engaging (based off memory), I really like the softer nature of the COP for longer period use.
Oh yes, it actually demands just a hint more volume than the X2, if that means anything. The COP is known to be absurdly easy to drive, which lets you know the X2 isn't demanding.