I still have an old HD650 sitting around, and I don't think the PM1 sounds anything like it except that neither offers sizzling treble. The 650s have a sort of closed in, claustrophobic sound and many details are missing in action,
and it's not the last word in speed, and the bass is moderately opaque. The PM1 is none of the above; it has much more definition and insight.
I do agree that the PM-1 is a step above the HD650 in nearly every conceivable way, but my statements about them being similar were in that they have a generally similar sound signature to my ears, and the PM-1 occupies the same spot in the industry that the HD650 had ten years ago.