Two months ago I would have said the B&O H6 for closed, and the Sennheiser 58X for open. I finally took the plunge into mid fi after doing some extensive research. Got lucky and chose the right headphones -
Audio Technica ath-Wp900 - Closed back / Portable.
Modded Koss Porta Pro (Yaxi pads + Parts Express headband and custom strap) and Philips Fidelio X3 would be my selection, and I just got the X3s in a few days ago...
Modded Porta Pro - warm, smooth, solid full-bodied sound with great midbass presence, forward vocals that are beautifully presented, and relaxed treble that is just enough detailed yet provide an absolutely non-fatiguing sound moreso than any Sennheiser. Simply the most euphonic and soulful sounding thing I've heard when paired with tube amp and the right tube, yet still sounds really good out of most anything.
Fidelio X3 - bass extension on par with the TH-X00 Purplehearts and though not as impactful /powerful sounding in subbass, has extremely tight and clean lowend with visceral snap and punch to it that is simply beyond most anything else. Reminds me of planar extension/speed but with dynamic liveliness and power. Vocals are very forward and present as well compared to any Fostex/Denon with very nice detail. Treble is up there like the Purplehearts but a bit less energetic. What makes it really special is its really natural sound yet also MASSIVE sense of space it gives to the music - everything has room to breathe and nice sense of reverb and depth though not particulary wide or best at imaging (TH-X00 Purplehearts and HD58X are better at imaging.)
Easy decision because I intentionally built my system around 2 headphones.
Sennheiser HD800S and ZMF Verite Closed. Perfect complementary headphones. What's more interesting is what is happening with the AMPs and DACs to get more diversity of sound.
Hugo 2 with Equalizer APO on my laptop can turn these headphones into anything from R&B Bass cannons to light and airy Mozart piano or violin concerto concert hall. Bottlehead crack for fun sound for these high impedance cans. After a year of tinkering, I am finally satisfied with the sound and flexibility of my system.
Took me a decade but this is the perfect pairing for my preferences.
Easy tuning adjustments for both of them so I get different sounds with different configurations. All I need. I guess...
10 years ago, came to headfi to find out if the dt880's were the one for me. They were my end game. 10 years later, i'm back, I find the closed back pair i've always been looking for after hearing the dt880 and this is why I would choose these two headphones.