Thank you Klipsch for giving us head-fiers the chance to help with the development on these new headphones. If any company can do an amazing job with headphones, it's an already amazing speaker company like you guys.
My thoughts on the HP series sort of reflect what most have said already. Comfort is a huge factor, so multiple choices in pad materials should be considered.
When it comes to sound, I have a lot to suggest. My stance is similar to rthomas, in that you guys should look into a reference tuning similar to the Sennheiser HD 650/600 for at least one of these headphones, maybe the flagship. I say this because it seems that most headphones that exisit give up a smooth, natural sound for other aspects such as in your face detail and driving efficiency. In my opinion, smooth frequency response and low total harmonic distortion are key to make headphones sound better. I would seek advice from Paul Barton of PSB speakers, as he has a good understanding of what headphone tonality should be like to most represent a high fidelity 2 channel speaker setup, and he has worked with big names in the field of audio science and engineering like Sean Olive. Many people including myself would love to have a headphone that is as close as possible to a stereo speaker experience. Also, making the headphone flagship an open back is a very smart move. I think the end goal at least for the HP-3 is to make it sound as open as possible, without affecting its frequency response negatively, easier said than done I know. You guys probably already have all this research done and I'm just giving you old news. Also, I have not heard your on ear reference headphones, which probably are a similar tuning to what these new heaphones will be. However, you guys are one of the few companies to ask for advice from us headphone enthusiasts, and I think that makes all the difference in the world.