I would agree with oqvist that the soundstages are very different from one another, and that might account for the "drastic" difference I have been hearing between the two recently. Ultrasone to any headphone seems to be a big leap.
Getting used to a headphone seems to be a double edged sword, like aimlink was describing. You know all the small details and subtleties, but end up either disliking the sound, or I will add becoming ignorant to its faults.
I have been enjoying my AKG more than I have in a long time, like I said. They are not laid back like senn, they are not bright like my SA5K or 325i... they are just nice and smooth. Relaxed and bit and balanced. They don't stick out in my collection as a "this" can as I stated before, but they certainly have their place.
Aimlink, I don't mean to be confrontational, but I would strongly argue that congestion is measurable in a headphone. Yes, by definition if a passage is busier, it will sound more congested, but how adept a headphone is at portraying that information varies greatly. A term I use for this is resolution. While this might not the audiophile accepted definition, resolution is the clarity of the musical presentation, in other words soundstage but with a few more criteria: So separation, placement, and independence of frequencies, if you will.
The 325i are quite like the SA5K, albeit of significantly lesser quality in every category (and quite frankly they should because the SA5K are more than double the MSRP!). They are very detailed, but have horrible resolution. You have to work to separate things in a song. It is very detailed and has lots to offer, but you are then blind to other parts of the song. Pro 750 on the other hand, whilst less detailed that he SA5K, are extremely resolute. You hear everything in a song effortlessly. Where the grado and SA5K have parts that are greater than the whole, the ultrasone has a sum that is greater than the parts. However, the amount of detail the pro 750 retain, while gaining an incredible advantage in terms of resolution somewhats defeats the purpose the detail in the 325i and SA5K.
I hope that makes some sense to you.
as always, theres my very long two cents

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PS, when can we expect impressions?