From what i have read the 400/500 are meant to be much easier to drive than the 5/6. If the 6's work well i'm sure it would have no problem with the 400/500 then.
The 400's are on eBay for $250 which caught my interest, i had no idea you could get a planar headphone for that kind of money, tempted to try them to dip my toe into the planar world. From what i have read i think the 500's would be ideal but they are a big jump up at $600.
Unfortunately there is nowhere in this country to try out headphones, i would love to hear the T1,HD800,LCD's or high end Hifimans to see which one works for me. Just too damn expensive to buy blindly.