Ah. The Primes are more revealing, more picky with sub-par recordings. Ont he warm side of neutral, I guess, with more even frequency response and much deeper bass extension. The 400i's are just fun. They sound great with all my amps, and all of my music. They're just missing that last 10% of detail and nuance that the Primes reveal. I feel like I have all the bases covered owning them both.