I’ve auditioned or owned all of the above.
HD660s is a bix more relaxed-sounding. The HD 6XX is produced by way of collaboration with a company called DROP, which as you may already know, produces some discounted audio gear. Manufacturers cut corners where possible, and then sell to DROP. They then sell the product to you...
I believe the HD650 is a bit better though, right across the board in terms of a more neutral sonic presenation. Most headphones will have a sound of their own; rather than be a chameleon that takes on the color of the source component/amp.
Headphones that don’t have a sound of their own are truly accurate. They will reveal problems in a recording; such as sharp or unpleasant peaks, yet with really good recordings can also sound a bit warm, without harshness or grain.