Haven't heard the K7xx but I do own both an HE-400i (same as HE4XX) and recently a Sennheiser HD6XX. I'll try to break it down to categories:
HE-400i: Generally flat with emphasized highs, bordering on "sparkly".
HD6XX: Flat with a slight emphasis on mid-highs, and a slight emphasis on the entire low spectrum.
HE-400i: Smooth, realistic, especially with good recordings.
HD6XX: What you'd expect from...