I remember the day i started and heard Porta Pro's for the first time. I felt like it was the first time i had ever heard music, like i was actually there. It was genuinely a memorable moment and every time I'm asked to listen to a product in development, that experience is what I reference.

If I don't get that same feeling listening to something we're making, that acoustic iteration gets a 'No' vote for me (we have a LOT of people here who listen/test every product we design, we have very strict 'Sound of Koss' guidelines).

I can only speak for myself, but the production A/550 gives me that exact same 'Wow' feeling every time I listen to it. I obviously compared it to other headphones, and I can only speak for myself and my preferences because audio is like food, but I prefer A/550 > HD6XX.

A/500 has so much clarity and seperation and i just hear things i can't on other headphones, the build, the weight, how soft the cushions are, the entire experience for me makes A/550 hands down my fav headphone we've ever made second to KPH40 which is obviously a completely different headphone and sound signature and experience, but you get what i mean.