Quote Matt Houghton:
I use my HD650s a lot now, though it took me a long time to get used to them. I find them very good for almost everything, and I'd recommend them to anyone.
I'd agree with that Matt.
My HD650's have been a revelation, and I do nearly all of my serious mixing on them now, but like you I do always double check the bass end on my little ATC monitors and inevitably find the loudspeakers have more bottom-end 'welly', so the HD650's are a little light by comparison. However, I'm just used to that now, so when mixing under phones I know what to aim for.
What I suspect the original poster may be suffering from is simply being able to hear everything so clearly. With good monitoring even a poor mix can still sound as clear as a bell, and it's only when you listen to cheap gear that you hear any level imbalances more clearly.
I always used to check my mixes on the small elliptical mono loudspeaker bolted into an elderly colour monitor screen used with my Amiga computer, because it was so good at revealing things like this (partly because a single full-range loudspeaker will let you hear the balance where multi-band 'hi-fi' loudspeakers confuse the transition from one frequency band to another).
As you say, once you've heard the mix problem you can still hear it through good monitoring. I know there are specific mono loudspeakers available for this purpose, but I really must try out a few cheap mono full range loudspeakers to find myself another 'el cheapo' reference