I have the Helicon 800's, and I have heard the Hyperions before. I am not an expert on the Hyperions by any means, but my impression of them was a far lesser speaker than the Dali's though I heard them with different equipment so as always you results may vary. I felt they had a bit of a shouty sound, kind of like some of the earlier Triangles, great on some music and bad on others. Also I thoguht the mids lacked a little bit of refinement, overall not a bad speaker but not one I could live with.
The Helicons are incredible speakers in my opinion. They are sweet yet extremely detailed, and have excellent imaging and decent bass. The Hyperions likely will have more bass output than the 400's, the 800's have excellent bass reaching quite a bit lower with more power and slam. The 400's if positioned properly will create an incredible soundstage. Some think they are bright but I think this is mostly positioning error as you do need to fiddle to get them right. They are probably the most placement sensitive speakers I have owned besides Vienna Mahlers (due to their bass output). The Dali's also in my opinion will sing with very high end gear, I think they would be your last upgrade for a while, I am not sure the Hyperions would grow with you as much so to speak.
One other word of advice, Dali's packaging is not so great, so if you go for the 400's used try to get them shipped strapped to a pallet via freight truck, you will thank me later.
