Massdrop offers the Massdrop x Sennheiser HD 6XX Headphones in Midnight Blue for $199.99 with free shipping. That's tied with our February mention as the lowest price we've seen. (It's also $168 under the best deal we could find for the similar Sennheiser HD 650 headphones.) Deal ends May 31. Based on the classic Sennheiser HD 650 Headphones, these extremely well-reviewed open-back headphones feature the same drivers and architecture, updated with a 6-foot detachable cable with 3.5mm plug and midnight blue colorway.