To be honest, I dont know if the HD650 goes lower or not - I didnt try with any music that has an organ (I will do so tomorrow). The HD650's bass has more presence, however. Think about how bass sounds in a concert - it reverbs off the walls and fills the space. That's how the HD650s sound. The Oppo sound leaner - that is likely more accurate to the recording, but, to my ears, the greater bloom of the HD650 is more lifelike.
Say you wanted to focus on a bass-guitar melody line, would you choose PM-1 or HD650 for that?
What I have in mind is, for instance, JJ Cale's Clyde, or, for much "thicker" music, perhaps something from the Grateful Dead, but I don't have a specific track in mind.