Longtime HD650 user here, who moved to Planar (Hifiman)
I started with the Hifiman HE-400s, and found it to indeed be a superior headphone to the HD650, but only marginally. Both are GREAT headphones, but, I preferred the tighter bass control of the Hifiman, and while I found that it had slightly less in the 'presence' band, the overall uptick in the treble, gave a greater sense of 'air' and space.
I am now moved on to the HE-560. WOW. This is a special headphone. Now, It certainly is 'brighter' and more open than a pair of HD650, but, I don't find it overall to be a bright headphone. I don't find it to be anywhere NEAR as lit up as say, a pair of Grado headphones. Actually, I think it sounds pretty much spot on in the treble, and even seems to have some 'flatness' in the upper mid band that I find troublesome to my ears... say, 2000 to 3000 hz. I like to call this the 'glare' range, and, this is where the Grado headphones about killed my ears....literally. But that is a story for another time.
So, I can also give a thumbs up to going Planar, specifically Hifiman Planar.
I should probably quality my observations by saying I am 100 percent tube amplification here. So, this certainly may be the reason I don't find the sound signature to be bright. The HE-560 seems to be an excellent match with tube amplification.
Oh, and speaking of that, while they aren't as difficult to drive as the HE-6, don't underestimate the power requirements of the HE-560. I did. I ended up needing to hookup directly to my tube amp's speaker outputs to get the drivers under control. The headphone output on my tube amp, which is a dedicated output with its own output windings, just could not keep up without massive amounts of distortion. But, now, I am sending about 15 watts of tube power into them, and all is great!! HEHEHE