Heard the Primes a few days back. Good god they are great. Reminds me of the Beyer T1s - they seem to do a lot really well, but nothing outstanding. Just a fantastic all-rounder for someone looking to own a single set of cans which they can use at home or on the go.
They are different beasts tho. I could see someone who listens to Jazz, Classical and a lot of vocal tracks preferring the HE-400i/560, while electronic and rock fans will like the Primes.
The problem is there is now a lot of competition at thie $1000 price point: Alpha Prime, PM-1, EL-8, HE-560, TH500RP, LCD-2, GS1000e... the list goes on. You could even grab a used HD800 or T1 for that price.
The hardest part I find is matching a specific headphone not only to your listening style, but also your equipment. My 'required' list for critical listening contains 3 amps, 2 dacs, and 6 headphones - just to get the most out of every type of music I like. No wonder this hobby is so damn expensive!
The He-400i is superb with the Lyr 2, agreed. I had borrowed some Orange Globes and the sound was sublime. I've got 75 Vokshod grays and some Amperex OGs on route for permanent use.
Cheers