Here's my 2 cents. I listen at work. I had the hd25-1's for about 4 months and just got the k271s a couple of days ago after hearing it and the Ultrasone HFI-700.
HD25: Great headphones. They are compact and sound great (especially with pop/rock) right out of a portable. Bass is really punchy, very fun headphones. The bass is tight and there's not much resonance. You don't get the smooth mids and highs that you do with the k271s. Not that the mids on the HD25 are bad, just not in the same class as the k271. If you don't plan to use an amplifer, get the HD25 and don't look back.
K271s: (Disclaimer: Remember I've only had these a couple of days. Also, they're probably not burned in yet.) Very smooth and nice mids! You really need an amp to get enough bass. Plugged into my home stereo they sound very nice. They have enough bass for me with a decent amp. The mids still sound fine from a portable, but you lose most of the bass and you have to turn the volume up considerably. With an amp the bass is not as tight and impactful as the HD25.
Ultrasone HFI-700: (Huge Disclaimer: I only listened to these for 30 mins directly from a portable) Lots of bass. Lots of treble. Did I say lots of bass? I really wanted to like these from what I had read online. Maybe with an amp they would have sounded better. For me there was just too much of everything. Too much highs, too much bass... It almost seemed to me that the details were enhanced. Again, I only heard these for a short period without an amp.
Sizewize the K271s are much bigger than the HD25. It's like bagels on your ears. Comfortable bagels though. The K271 do get hot and I get "headphone hair" that I didn't get with the HD25. The Ultrasones are halfway between, similar in size to the Sony V6/7506.
I was looking for something that would be better for classical music, maybe more musical and sophisticated in general. In the end the smooth mids from the K271 won me over. Now I'm looking for a inexpensive amp to use with my K271 from a portable. Any suggestions?
I hope that helps.