Not very good with explaining what I hear but I'll try my best. I can't properly place the TH600 and the other two because at the time, I only ran it through an X-duo portable amp but, one thing for sure, the bass in all aspects beats both the HE-500 and DT 1990 pro. The 500's though have great bass if you're not comparing them to one of the most bass focused Cans on the market from what I've heard. The 500's bring great impact and substance with better midrange and detail. The highs in the 500's a great but nothing fatiguing but from what I remember, the 600's kicked ass for female vocals from the one song I always played - Thefatrat - Monody. The 1990 is the least "fun" of both cans but that isn't something negative. It's extremely well balanced. It's not lacking on anything to be honest and is one of the best all arounders from what I've personally heard listening and what I've seen almost everyone that has it say. I did research on them for about more than a year before deciding to pull the trigger... I don't regret it one bit much like I will always love the 500's even after owning the HE-6's which are supposed to be much better. Since I didn't have a good setup with the 600's I can't say that this ranking is fair but it's HE500 = DT1990 Pro > TH600's. The first two do a great job of everything without exactly having one particular weakness. The Mids in the 600's arent the best but, I don't think it's what people who buy Fostex are looking for anyway when buying them. It's interesting you stated again that it's about 90-95% of the 900's. I'll have to consider that as well since I now have a decent amp dac setup.