Frankly, I'm not too worried about this particular reviewer (aornic) describing the X5000's as not being as impactful or not as dynamic as he'd like. I'm using a 1970's-era SONY STR-6055 as a headphone amp, run through a "Can Opener" directly connected to the speaker taps. This essentially acts as a step-down transformer to bring the voltage down, and seems to provide a bit more smoother presentation, compared to plugging in headphones directly into the headphone out of the SONY. It isn't a huge difference, but enough to notice, as I find this configuration preferable to simply plugging in my headphones directly to the headphone socket of this receiver. The headphone out has a relatively high output impedance (which I think would match up well with the X5000's relatively high impedance). So, if I had to bet, I'd be willing to say that my amp would probably provide a bit more punch than the average dedicated headphone amp. So, as far as that goes, I have no worries. Plus, in considering this particular headphone, I'm interested more in what this headphone is capable of delivering as far as microdynamics, rather than macrodynamics (slam).
I already own a "fun", less-refined, and "U-Shaped"- sounding headphone that provides bass slam (the T90). If the X5000 doesn't bring quite the low-end punch that the T90 does, but makes up for it in additional refinement (bringing forth more of the harmonics and overtones, etc. of acoustic instruments) then for me it would be an agreeable trade-off. But, I would be willing to bet that I'd get a bit more bass impact out of this headphone than the reviewer did, considering what I'm using as a headphone amp. (I can't say that for sure. I don't know. I'm always careful about commenting on gear that I haven't heard). But, that particular reviewer's comments about the bass performance of this headphone didn't mean much to me. He seemed to only be concerned with the bass impact, and that's about all he said as far as how the bass presentation of this headphone sounded. Well, if you want a headphone that hits hard in the bass, you've got plenty of choices, these days. I'm looking for more than that, as far as how the overall bass and low-end is presented. I'm not picking on this particular reviewer, as this was just a short, first impressions, "better than nothing" quick take on these headphones in a noisy environment. But, he may listen mainly to electronic music and was disappointed in the lack of bass impact.
(Which is what I suspect). But, again, I'm willing to give him the benefit of the doubt, as this was a spur of the moment and rushed, initial review.