Yes, that metallic edge is Ultrasones trademark identifier, I think. Luckily, it doesn't bother me but I totally understand your feeling after trying many of their models.
Their 2900 sounded particularly metallic to me, which was why I sold it. More so than their 2500, which I preferred in direct comparison. I'm not sure if it needed more burn-in or if it was just the headphone itself.
I'm guessing it's just a particular characteristic of titanium/ titanium plated drivers.
I think it might simply be the tuning that is the issue and not an intrinsic property of titanium coated diaphragms - although this is just a guess. The reason I guess that is because most headphones with extremely forward highs I feel comes off metallic as well.
Also, I have easily over 2,000 hours on my pro 900s and have had them for almost 2 years now with heavy use daily (average of 5-6 hours a day), so I am sure it is not an issue with needing more burn in although I have to admit they did tame a bit after 50 hours of use. Burn in with these definitely works, but not much more past 50-100 hours.
I found that the best result for the sound was replacing the stock pads. It dramatically opened up the soundstage so much more that sometimes I have to take my pro900s off to make sure the audio isn't playing through speakers. It tames the bass and highs a tad too so it is fantastic. I am currently using Edition 10 pads on my pro900, but I would assume any pad that has ventilation holes (such as hifiman focus pads or Edition 10 pads) or very thick pads that dramatically increase the distance between the super-shallow cups and the outer ear (such as Audeze or Alpha Pads) will work like magic to get the ultrasones to amazing heights.