Thanks for your review/impression.
I use the RS180s and have owned the HD518s, HD590s and AKG K240s.
I would say that "flat" headphones cannot have the same bass response of a pair of speakers. Due to popularity in the last 10 or so years of music with heavy bass, the headphone manufacturers have had to boost the bass response, in order to get popular genres to sound reasonable. That automatically obscures some of the higher end.detail.
I could verify this with my 518s by putting my hands over the grills in such a way as to diminish the bass response - which instantly improved the mid-and-treble detail. Various other tricks - like 555 mod - do similar things.