Detailed comparisons are reserved for my Head Gear reviews.
However...
Let me state upfront that closed-back headphones don't really compare to open-back. Open-backs have a way of always sounding better, more natural and spacious. The HD650 (or my HD6XX) is superb even among its design brethren.
Of course, there are situations where closed-back are needed. Many, many situations where the isolation is a necessity.
The Meze is not the HD650. Not only is it closed-back, and therefore doesn't belong in a comparison, but it's also a super high efficient "mobile headphone". These types of headphones tend to lack some of that weight you find with a beefier set of cans that hunger for a desktop amp just to sound right.
That said, the Meze has more note weight than I expected, coming from the Momentum 2.0. It has just a tiny bit more treble than the HD650, making the vocals sound less lush, but utterly clear. Not the aggressive detail clear that bright headphones create. Rather the smooth, rich clear that comes from a marvelous balance of tremendous bass, quantity and quality, and clean, revealing highs. The vocals just sit there benefiting from the wealth on both ends, giving them something special that's hard to describe.
It's easier to describe the HD650's vocals. They are lush. That's the word everyone uses, and it suits them. The 99 Classics are more nebulous, but just as remarkable.