To me the NO is a gentle \-shaped sound. Bass is warm and extended with good impact. Mids are a bit recessed from there, but lush and well-detailed. Treble drops off even more, giving cymbals a solid fundamental tone, but less shimmer and air. Surprisingly, they have great soundstage. I'm being brief here, because you own them.
By contrast, the 900 has huge impact and presence in the bass, but it's also controlled and textured (though not to the level of a good planar). It's addictive, thrilling, and visceral. To me, it's the absolute pinnacle of dynamic driver bass, and the NO simply doesn't compete in this regard. The mid-bass, surprisingly, doesn't bleed into the mids very much at all, certainly less than the NO. Mids are clean and clear, with detail and realism. In my opinion, they don't live up to the price-tag, but they are still quite good. They are, of course, a bit recessed given the hard V-shape of the sound signature, but they are present. If that were all there was to sound, the 900 would be an easy recommendation over the NO. The problem is that the treble of the 900 has a peak in it that is absolutely searing. Where cymbals tend to sound like a cowbell on the NO, they sound like water being thrown into a hot skillet on the 900. Certain frequencies border on being unbearable. It needs some kind of tuning pad to go along with it. Unfortunately, measurements do not do justice in revealing this treble issue. Even if you can handle that treble, the result is still a wildly unbalanced sound signature. It has it's charms to be sure, but it is certainly not an all-rounder as it tends to make all music sound like EDM.
