My sealed can collection includes K271, A900, CD3000, HD280 and HD25, as a sealed can does indeed come in handy from time to time.
The 271 serves as my office headphone, largely because of its auto-shutoff feature. It's a good all-around headphone, but it lacks something. I can't pin down what it is in frequency-spectrum terms; all I can say is that despite being very accurate-sounding, it doesn't really involve me very deeply in the music. It's rarely hard for me to take it off when the phone rings.
If you like to rock out, the A900 is a very good sealed choice. Fun-sounding, lots of bass, easy to drive. I find it too heavy in the bass and a little too strangely colored in the mid/vocal range for all-purpose listening, but it grooves really well with instrumental jazz and rock.
The CD3K is sort of semi-sealed: it doesn't leak (or at least leakage is pretty minimal), but it doesn't isolate you from outside sounds to any useful degree. Of the closed cans I've listed here, it's my favorite. Very balanced, with an ultra-clear hyper-detailed presentation that some have compared to the SA5K. (Its looks, however, are not comparable to those of the SA5K, if that's what's really grabbing you.) If you want a sound signature different from those you're experiencing now but nevertheless extremely high-quality and in a non-leaky form, this might very well be your choice.
I don't like the HD280 much and keep it around only for its high isolation. This is the can I would lend out at my old office when someone cam around and said, "extended conference call in the cubicle next to mine, help me."
The HD25 is fun, easy to groove to, sealed/isolating, portable and easy to drive. If that covers some useful extra bases for you, you might consider it. Its bass-heavy presentation tends to put vocals a bit in the background, though, and IMO it's not useful for classical at all.
I'm afraid I can't speak to the DT880 or SA5K. Good luck with your choice!
