That's reassuring, I do notice the sibilance in my pair but it's not intolerable by any means. I listened to a friend's dt880/600 briefly before pulling the trigger on mine. I might have to borrow them for a while once my pair are broken in and compare.
The O2 is touted as a neutral amp that will pretty much just let you raise the volume - my ears confirm this so far. Pairing it with the dt880s, which are also quite neutral, might sound flat or unappealing for many people. Do you have another pair of headphones? If you do, it might pay to try a range of amps and see if you'd prefer a neutral one or maybe something with more colour.