They are a kind of W signature, but to me they do subbass impact and treble energy better than their mids... definitely more subbass presence than any other headphone I've heard at normal listening volume (I don't listen loudly...) With driver housing damping, certain pads, and custom attenuators, they have very nice and tight bass which doesn't walk over the other frequencies.
Their lowend is very much subbass, rather than midbass, focused:
The combination of the Purpleheart's subbass presence, full bodied sound, brightness, punchiness, relatively clean sound with very good imaging and separation, and overall energetic sound really make it one of my favorite headphones. And with the right tubes in a Project Ember it absolutely comes alive compared to any SS amp and is much easier on the ears for long listening sessions. Porta Pros with Yaxi pads are a great complimemt to them, being much more midbass and vocal focused, warmer, relaxed/not bright, and rolled off at both ends.