The Sundara is basically a little tweaked HE400i, so not worth investing the extra money.
Dont know whether u like planar bass or not, but the 660s has a sub bass rolloff.
But even if Sundara's bass is more linear and detailed, its kinda thin sounding, just lacks weight and dynamic punch.
Another thing, that Sundara's treble is a little too hot, but that probably not a problem for you, since the he400i even hotter and u like that.
IMO, the biggest selling point of the Sundara against HD650/60s, is the really good soundstage and imaging, since the Sennheisers really sucks in terms of soundstage and imaging. Maybe the HD600 not so.
Ah and another thing, the Senheiser is much more comfortable. The Sundara is much heavier, while the cups big and not swivel. But that maybe ok too, since the He400i quite similar design.