HD 600

Tubes do help the HD 800 S, it becomes more "HD 650-like" by adding some warmth and taking the edge off in the treble. Though it's a much "Softer" presentation than the HD 650 below 4kHz, as shown below. Though, the maintenance needed to upkeep tube amps can be annoying if you're used to solidstate.

I’d say if you’re one of the vast majority of people that want headphones but under the Sennheiser brand, Momentum 4’s is the way to go. But if you’re one of those who actually really cares about sound quality more than anything in the world? HDB 630. I own the 630, it’s the most versatile headphone