Sure I will do what I can. No, I was not tempted in the least to get the premiums. The price premium is just too much for me. The 770's and 990's sound very different to me. The first thing I noticed when I listened to the 990's the first time was the open design. I could still very much hear all the background noise in the room, and surprisingly this is why I wanted them. The 990's have more prominent highs and the bass seems slightly louder than on the 770's. The 770's have more prominent sub bass though, but the mids and highs seem slightly recessed when compared to the 990's. The 990's sound like a more balanced headphone when you compare them to the 770's. I should also mention that the 990's I received in a trade had been modded as seen here:
I have not yet opened the headphones to reverse this mod to see what they would sound like by default. I would describe the 990's as having slightly prominent bass and highs while not being congested in any area. I would say at this point that I enjoy the sound of the 990's better, though I am still exploring and getting used to them. The O2 combo is imo the perfect dac/amp for these headphones. The O2 has a well documented design that adds nor takes anything anyway from the 990's. I use the 1X gain setting on the O2, and the comfortable listening level is about 40% on the volume knob. I hope this helps as I am not very good at describing how headphones sound.