I currently own a pair of HD650's and X2's and can hear them side by side.
Here my impressions:
The X2 definitely sounds more sibilant, especially in songs that tend to a more sibilant sound. For my taste the highs are a bit overtuned , however in some songs this creates a more interesting and engaging sound. In others however, it leads to a mess with uneven treble. The 650's dont suffer from that problem at all.
The X2's bass is definitely more prominent and also reaches a bit "deeper" - However the HD650's one is more punchy. In direct comparison the X2's sound can sound a bit muddy - not to a terrible level and only in direct comparison, though - sometimes the X2's sub-pass kicks a bit harder leading again to a more pleasing / engaging and "fuller / more spacious" sound.
The X2's soundstage feels to be wider (but not by a huge margin).
The X2 is a bit heavier (but doesn't feel so on the head) , the clamping force is less and the pads are more cushy (I like wearing him more). Build quality wise I also prefer them over the plasticy HD650.
I say : If you can handle the peaking treble / sometimes sibilant sound, go for the X2. Here in the EU they are less than half the price of the HD650s.
The HD650 is like the best guy in school, he does nothing wrong and always sounds pleasing, however also can sometimes be a bit boring and uninspired. The bass quality is a bit higher / tighter, but the bass quantity definitely a tier lower compared to the X2.
Overall I think it is the better allrounder for music. Is it worth more than double the price of the X2's ? Nope not at all.
Maybe also have a look on the X1. They don't suffer from sibilance and I feel like they sitting directly in the mid of those 2 (bass quantity of the X2's with a little less sub-bass, tamed highs, good soundstage), unfortunatly without the tight bass of the 650's. They are my daily driver and always pleasing to listen to.
Also tried the AKG K712 pro - Very good highs and mids but even less bass than the 650's. I would prefer the 650's over them.
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Not trying to disagree with you: what you hear is what you hear. But I just don't hear sibilance in my X2s...not to any real/significant degree. I just cued up a number of cuts that are borderline bright, depending on electronics and headphones. And through the X2s, they're just not bright, neither vocalists or instrumentals. In fact, the bass and soundstaging are so fine that I keep forgetting my little "mission" (find the sibilance in the X2s) & get sucked into this or that earhole.
Maybe it's my electronics, all carefully chosen to avoid unnatural brightness (to which I'm extremely sensitive) and give me tonality that I hear in live music. Right now I'm listening through the Violectric V281, which is not inherently bright--though it is almost absurdly dynamic, and those shocking transients can sometimes be mistaken for brightness (they're not--in fact, the notes roll in with a wallop from top to bottom of the frequency range). The V281 is a bass monster, and the X2s are right there, keeping up. My DAC is the non-oversampling R2 R design from Audio GD, the NOS 19. It's the least "digital" DAC I've ever heard: zero glare or tonal thin-ness; instead, a full, organic, relaxed sound with excellent, slightly "wet" sounding bass (ie, not dry, cut-outs of instruments, but full-bodied & impactful).
The X2s sound AMAZING through this equipment. They also sound pretty amazing through all my other HP amps, also quite good through my favorite el cheapo delta-sigma DAC, the Peachtree Audio DAC iTx (transients are sharper than w/the NOS 19, for sure...but the X2s still don't really sound sibilant/bright).
(I'll have great difficulty getting the X2s off my head tonight)
Anyway, I'm hearing what I always hear from the X2s...rich, expansive soundstaging, the kind sealed designs never quite get; a relaxing, friendly sound with lots of bass, mid-bass, and lower midrange energy--and little or no glare, brightness, sibilance, etc. I'm not sure if the X2s are V-shaped, U-shaped, or just a little tipped up in the bass and slightly down in the mids. I've surely heard headphones sound "bright" (just not these).
I've heard other headphones that do "technicalities" better than the X2s--but so what? For $200 they're not only a screaming bargain, but they suit my sonic preferences as well as any component I've heard in ~40 years of audiophile wandering.
