My ears are already watering 

I have something similar with my HD650 - a simple 40hz low shelf filter really helps recover subbass in my opinion.

Honestly, it seems to me that diminishing returns in headphones hit a lot sooner than in speakers. While I'd much rather spend $50-$300 in headphones, I've heard $10k speaker systems that just blow any headphones out of the water, it's not even a competition.   I think this attitude is really...

I don't think that's necessary. Sennheiser has a history of keeping a working formula. The HD580 was replaced by the very similar HD600 and it has been in production for 20 years now. I doubt they would discontinue the HD600 unless they could produce a direct upgrade at the same price. It still...

Superlux HD668b will always hold a place in my heart, it made all my past headphones (Skullcandy, Razer) sound like I had cotton in my ears in comparison. I doubt I'll ever get such a drastic increase in sound quality ever again... maybe when I finally get good speakers.