I have both pro 900 and x1. Two of my favorites that I have kept after buying and selling MANY others.
The pro 900 is an EDM headphone. It has amazing fast bass, and sharp, clear clean highs. It's warm in the bass and bright in the treble at the same time. I've found I like to EQ the very top highs down a bit, to give it a touch smoother top end. But I also listen often without this EQ.
But yes, this is a sharp sounding headphone, that puts lots of detail out. It's an incredible headphone for EDM, and anyone who likes the genre MUST try it, IMHO. The tightness of the bass and punch is just fantastic.
The X1 is a great headphone as well. Comfort is excellent. Bass is deep and enjoyable. Highs not as sharp and extended and emphasized as in the Pro 900. Again, as great headphone for EDM. I actually use my Pro 900 short cable with the X1. It's my favorite cable I've found for the X1... the X1 also has a very nice soundstage.
The X1 isn't as fast, sharp, aggressive, and intense as the Ultra-sone. It's a smoother and more relaxing ride.
For bass-heads who can tolerate somewhat sharper highs, Pro 900 is a dream come true. Probably my favorite HP I've ever tried, including ones costing many times more. It is a unique sound, but boy is it fun.
The X1 is very special as well. If I had one annoyance, it flattens my hair (with that massive headband!) in a way I really hate, and I always have to adjust my hair after I'm done listening. But sound wise, incredible value for the price. I really think it's underpriced for the quality you get - both sound and comfort.
These are both headphones with a bass emphasis. The 900 has the highs emphasized.
Try them both, both are excellent and well worth it. Not to mention, they aren't at all hard to drive either.
How much bigger of a soundstage does the X1 have compared to the Pro 900 even though the Pro 900 have S-Logic? I imagine the Pro 900s still can't compete since they are a closed back.