K701/K702's will have the most neutral and be the most detailed/analytical/transparent or what ever you want to call it. Bass really is its weakness. I used to think that this was not that big of an issue. I recently acquired a set of Stax SR404/SRM-313, which I believe still doesn't provide as good of midrange and upper midrange performance as the K701/K702, but quite a bit better in terms of bass and treble extension and quality. To be sure, the K701/K702 still offers a more realistic rendition of instruments and voices (to my ears anyway), but the SR404/SRM313 combo is about its equal when it comes to detail and resolution. The better bass and treble is really apparent and I find my self listening preferring the SR404/SRM313 for most. But every time I put on the K702s I marvel at just how good these phones are for what they cost.
In comparison to the K701/K702s, the DT990 is much less neutral and more "fun". It's got fat bass and very aggressive treble, therefore the midrange will sound recessed in comparison. If the K701/K702 is an accomplished gymnast, the DT990 is a "coke bottle figured" model. It doesn't reach the same level of detail as the K701/K702s, less separation of the various layers in the sound it reproduces, but makes up for it by delivering a response curve that is more energetic. The boosted bass and treble are of very good quality, especially the treble, where it only becomes sibilant and objectionable on certain material. DT990 is also great for older recordings where the treble is rolled off to begin with, resulting in a pleasant "remastered sound", if you will. One potential downside is that some times these older recordings have quite a bit of background noise, which is exacerbated by the treble response of the DT990's. In all, I find the DT990 to be very satisfying with certain types of music, not so much with other types.
The HD650's that I have exhibit neither the detail or neutrality of the K701/K702, nor the fun factor of the DT990. My faint praise for it is that it's a perfectly capable pair of headphones and the sound it reproduces is of good quality and not objectionable. While it doesn't suit my particular tastes, it has plenty of supporters here and elsewhere.
If there was *any* way possible, I would recommend that you listen to these headphones on your own to decide which ones you want. One possible way is to "rent" a set by buying used through this forum's trade/sale section. Most of these headphones hold their value well and you can probably buy a set in good condition, listen to it for a couple of weeks, and sell it to get nearly all of your money back. You can then buy a new pair of which ever headphone that best suit your tastes.
Jack