We will have to disagree here. Take what you said about the hd700 vs the he400 and reverse it. I wonder if you didn't type that backwards by mistake. The bass on the he400 is the crown jewel of that hp and to my ears is several leagues above the the he700. I found the hd700 harsh, strident and overall unpleasant to listen to. And yes I listened to it on some very good amps.
The he400 bass is some of the best audiophile quality bass in the game and at $300 you won't even get close with anything else.
I haven't heard the HE-400, but if it's anything like it's sibling the HE-500 then I must totally agree with Matt about the HD700. It is IMO the most coloured HP in the Sennheiser HD 500+ lineup.