People complaining about PC360? Have you HEARD Sennheiser products? They are almost always very linear, warm, and non-fatiguing. They're not going to scream excitement. If you prefer the AD700, it's because you pefer a strong treble emphasis. They're complete opposites of one another. If you compare directly, of course the Senns are going to sound dull. Same way as any warm, smooth headphone will sound dull when you're used to bright headphones.
Honestly I just find Sennheisers in general boring regardless of them being dark or bright. Some reason they just fail to engage me, to me they lack what I call the euphonic factor. I personally can't agree with the notion that any smooth and warm headphone will sound dull if one is used to bright headphones because that hasn't been my personal experience.