Been learning about headphones and recently acquired AKG-K240s, Sony MDR-7506, Sennheiser HD 558, and yesterday, a pair of Sennheiser HD 555s arrived. I'm listening to them now. I like the open back sound. From reading about all the different headphones, how they sound, the prices, peoples opinions, I had about decided that the best, for me to shoot for, would be the Sennheiser HD 600s. Then, yesterday, I came across this headroom headphone graph comparison tool. Not being real familiar with translating these graphs, it looks to me like the HD 555,558,600 and 650, are all quite similar. The main difference that I see, is that the HD 600 has more treble than the others. Would it be correct to say that the difference in sound of the 555.558. and 650 would be negligible? The HD 600 shows noticeable more treble, but would that translate into significant improvement in sound over the others? Significant enough improvement to plunk down an additional $300? What says the people who have these? Or what say those with an opinion?