I'd agree, HD600 is hard to beat for the price you can grab a used (basically new) one on the FS forums here ($250).
I spent a short bit of time (an hour?) with the HE400 and it struck me as a very fun phone; in the same vein as the old Denon D5000. If you were into like dubstep, hip hop, or basically any bass centered genre. They seemed to have some real punch and sparkle to the sound.
I also spent a weekend with the HE500 and I disliked it very much. So much so I instantly sold it. I know this is contrary to popular opinion and it's a highly regarded headphone here but I'd pick the HD600 over it any day (regardless of price; of which the HD600 can be had for about half the cost). Perhaps the HE500 needed more than the ~1 watt my DACmini PX can put out to really sing, I never did try it off the speaker taps.
....but anyways, I like the HD600 more. Though I think all 3 headphones are competent, it comes down to preference.