The T60RP's are beautiful, full stop, the different wood texture on each cup. The headband, even with the added comfort strap, doesn't come close to the Amiron or Nighthawks. The first thing I noticed is how tight and controlled and controlled they over the entire frequency range, if not fast, the cymbal and string detail and texture is a notch beneath the DT1990's, but better than the Amiron. The sub bass extension is the worst of everything I've compared here, but that's not to say they lack bass, it just doesn't have the impact that the Nighthawks have...these aren't for bass heads. The imaging and sound stage work the best of the bunch, it feels organic, and the separation is amazing.
After 30 minutes of listening, these are the headphones that most make me want to reach for the EQ. Throwing 5db at 30hz, and 10db at 20hz made them a lot more enjoyable for me in songs where I'm used to feeling the bass.
Moving back to the Amirons, they sound competent on every level. Definitely the fastest of the three, and the most well balanced....dare I say the most boring? Then I went back to the T60RP, it's the most transparent of the group, the "planar slam" is there in the sense that you feel the bass, but the extension and quantity isn't, these are the flatest. Switching back to the Nighthawks is jarring, their non traditional frequency curve takes a good 15 minutes to adjust to. The sound muddy at first, but after your ears adjust all the detail is there and the bass has speaker like impact, but the character of the upper mids make rich male vocals sound flat and boring.
Finally comparing to my trusty Fidelio X2's. The Amirons feel like an improved version, with a slightly smaller sound stage, less bass quantity, but lower extension and much much faster and completely unoffensive in every way. The T60's sound lively, the more I listen to them the more I like them, definitely the best for critical listening. Once again, the Nighthawks feel unique, and after adjustment sound amazing, in a hard to describe way, while not as fast as the Amirons, the bass is the best of the bunch as far as extension, quantity, and feel. The vocal texture in the uper mids can be brought back through EQ with a 3db increase at 11k and 18k respectively without damaging the overall sound signature.
Really not going to be easy to choose which to keep.