I have both the Sennheiser HD 820 and the Sony MDR Z1R. You might want to consider the Sony MDR Z1R is you want more slam Aka bass.

I had the Sennheiser HD 660 headphone and I was really disappointed. The didn't sound good compared to my HD 800 and HD 800S. 150 OHM VS 300 OHM

Agreed 100 percent! The 6.3mm flattens out the sound on a balanced amplifier but if you used them on a ALO STUDIO SIX or a Enleum Amp 23r they would sound really good!

4.4 mm pentacomme and XLR sounds better than 6.3mm unbalanced connector.

Sony MDR Z1R is a really good closed back dynamic driver headphone too! I have both headphones and enjoy them equally.

The Sennheiser HD 820 are great for gaming but you can't use them in public they leak sound and people complain.

I agree the Sennheiser HD 820 does have a good Soundstage for a closed back headphone. I also have the Sony MDR Z1R headphone it's soundstage is narrow compared to the HD 820 but the Sony is a enjoyable headphone to listen to.

I have both the Sennheiser HD 800S is a better option. Reason mainly because of the Sennheiser HD 800S Soundstage is better than the HD 820.

I would go with that and the IFI purifier

I would go with the one that has balanced 4.4 mm connector

Ifi DIablo is what I use with my HD 820 headphone. It has enough power to drive the headphone along with a 4.4mm balanced connector

I did both modifications. I had both the HD 800 and HD 800S. They sounded similar afterwards

I have the Sennheiser HD 800 headphone with the anaxillus and super dupont resonator modifications do you think it is worth picking up a pair of Sennheiser HD 800S to use with my ALO STUDIO SIX Amplifier?

Studio Six and Sennheiser HD 800 great combination.