the Parterre has clearly more trebles than the 334. I'm talking perhaps around 10kHz(I've checked precisely but forgot the values and don't have them with me to verify sorry). cymbal mostly have a bigger presence on the Parterre.
about burn in, I'm not a believer, but those I have were really killing me when I go them, I even had to spend a few days without music to get rid of some nasty tinnitus I got after 3days of Parterre(3intensive days I must say). they went on a big journey in many ears for 4months. and now, if they're still too bright for me(I'm a hd650 guy), they stopped wrecking my head so I guess something might really have happened. I feel like I could live with them and be happy about it with just a little EQ to reduce the 3khz and something on the cymbals when listening rock (on classic and cymbal-light musics they "rock" as they are ^_^).
take all this with a lot of salt as those are my feelings taken months apart from each other. for reference, the HD800 is also wrecking my brains and they are perhaps the best headphones I've ever heard. it's just not the kind of signature that helps me spend 6hours with music without losing a screw.