You don't typically see new releases from well established manufacturers on Massdrop other than collaborations. Sennheiser gets awfully touchy on pricing, especially with new releases. Hell even older releases... I bought my HD650 for 319 about a decade ago (it launched in 2003). It was the first time I had seen it sell for less than $350. For the next few years after that I didn't see it below $450 as Sennheiser cracked down on their MAP. As Massdrop has a manufacturing relationship with Sennheiser I don't see them skirting the MAP with Sennheiser products like they do with some of the other drops (where you only get the discount once you join).
TL:DR - Don't look here exclusively, you will probably see it cheaper elsewhere first.