I contacted Hifiman customer service about this issue, and they responded within an hour offering to ship me a longer head strap for $20, plus $10 shipping! I'll report back once I receive the strap and let folks know how it works out.

Greetings! I recently acquired a pair of Hifiman HE-560 V2's and I love them, except for one thing... The head "strap", for lack of a better term, is too tight for my gigantic head. I also have a pair of HE-400i's, and they fit like an absolute glove. I want that same fit for my HE-560's...