As are ears. Can vary slightly by source as well. But for example, I actually do consider the K7XX to be neutral as I do the HD600. But for me, they are different sorts of neutral. I actually don't feel an elevation in bass like you do, in fact, it seems overall flat to me plus a very tiny bit on the bright side. Going back to my
different kind of neutral point, I feel like the K7XX is the ever-so-slightly-skewed-towards-highs neutral while the HD600 is the ever-so-slightly-skewed-towards-mids neutral. It's why I personally prefer the HD600 as I like my mids a little bit more. I also felt like on the setup I listened to the K7XX on, the HD600 has more bass presence as well.
Overall, I will suggest that anyone should take every consideration we've all written here with a grain of salt as YMMV. Luckily, the one thing that doesn't affect is our ability to love listening to music, be it with these headphones or other headphones
I personally don't think the HD650 is a good choice for a non-dark headphone, but as I said above, YMMV. It's entirely possible for the HD650 to not sound dark at all if one has the right source or has been listening to darker headphones in the past.