Another update to my list. Not listing my DT 480 as it's not being used much currently as I'm unsure where to go on the modding at the moment, waiting for inspiration to hit.
1.) Beyerdynamic DT 831(closed, modded): Reverted it back to closed-back. The midrange on this thing is glorious, it's so full, musical, clear, and euphoric and doesn't seem to have any dips or accentuations. Fantastic body, dynamics, speed, and imaging. This headphone is the definition of excellent PRAT, it's a bit too addicting. Like any transparent closed-back it doesn't sound closed at all. This is the most transparent headphone on the list.
2(tie) Beyerdynamic Amiron Home(stock): My longest owned headphone, a bit laid-back in the mids but I can listen to these non-stop all day. I can always go back to these and I never grow tired of them. A bit on the u-shaped with an overall softer sound. More resolving and detailed with better dynamics than the HD 650. Very transparent.
2(tie) Sennheiser HD 650(stock): Newest addition to my headphone family, the HD 650 is definitely a legend for a reason. Very smooth and full sounding with some mid-bass accentuation. It's pretty obvious this thing scales-up with gear like a monster. Probably the best tuned of my headphones.
3.) Beyerdynamic DT 880 Black - 250 Ohm(stock): This headphone has grown on me, a bit on the lean side at times but doesn't really do anything wrong and very uncolored, sounds a little too damped at times. Still slightly unsure about it.
4.) Sennheiser HD 660 S(stock): I used to like this headphone more, but honestly it seems to lack a bit of transparency and has inability to scale up much as it's predecessor that prevents it from being truly great, the HD 650 ruins it for me on my system. I understand what Tyll was talking about on these now.
5.) AKG Q701(stock): If modded will be tied for number 2, but trying it out currently as stock. Has a lot going for it, but unless modded the upper mids are just not to my liking.