Edit: They're not night and day but if you like to keep things away from the basshead stuff and are more used to somewhere in the middle (neutral/natural) you will find them at opposite sides, sounding vastly different (with stock tips, I've just found out that I like my Re0 with comply tips which kind of mellow out the upper range a bit for a more relaxed listening).
Stock UE900 are quite bassy, veiled mids and smooth highs. While some like that, I don't. But put small auvios on them, shove them in deep and use the airline attenuator with 2/3 blocked pinholes, oh boy are they different. If you just happen to own a C&C BH they can be really versatile. Getting almost the tonality of the Re0 but with slightly more bass, less harsh upper mids and quite similar treble (smooth but very extended). But with better separation and layering. But I like the UE900 for being quite versatile. Block pinholes, use an adapter, use the mic cable. They're quite awesome. Btw. Sorry if I'm not making any sense.
Edited by MoonYeol - 9/9/13 at 9:57pm