Not sure we can agree there my friend, I blasted some very aggressive Chevelle, Hats Off To The Bull album with the PM3 and it was extremely engaging, same with Tool, Sevendust, Live etc. It just goes to show that nothing is an absolute truth. I think the PM3 takes a layer of noise off of rock. Rock is loud, but generally it really doesn't sound good and despite the fact that the artist may have liked the over-presented sound of an over-driven guitar amp mixed in really loud with a biting edge, that is no guarantee I will like it. Hell Metallica loved that horrible mess St. Anger and they could barely believe others didn't love it as much. Anyway, I digress, for me the PM3s help rock out, but as you did note that is highly subjective. That isn't to say that I can't at times enjoy a livelier rock headphone, hell I started out with Grado's, but for an easy to like, multi-genre headphone the PM3 is pretty solid even if it does soften the edges a tad.
Think we are actually agreeing and then it's down to personal preference. The Oppo does soften the edge a bit, and nothing wrong liking that. When cd first came out, I changed my whole hifi to try and achieve that! Now, I'm leaning towards excitement, ears probably losing those highest frequencies, and I like Gradi 325s. It's all good, so many choices in headphones, and now DAPs too.
On the mojo thoughts above, I would venture it has little sound signature of its own, a very natural sound. Whether everyone likes that is due to what they are used to and coming from, and what equipment accompanies it. I don't think it's sound can be beaten at sensible money, but still interested in an additional stand alone DAP. I heard the QP1R at London Canjam and it was best DAP I'd heard, above A&Ks. But aim to hear this Onkyo before making a move.