Headphone Reviews

Sound Quality
The HD 25-1 II has a lively, articulate, and well balanced sound. Bass extension is good for headphones of this type (as long as the seal is good), and the bass is tight and clean. The mids are well proportioned and nicely integrated with bass and treble. The upper mids and treble are slightly forward, and the highs can be somewhat grainy and hard sounding at times, though not excessively so. I would characterize these cans as having a very good "pro" sound.

The dynamics are excellent with these headphones; soundstage and coherence are good. I was pleasantly surprised how good they sounded directly out of my iPod, playing plenty loud and clear. They improved with better amplification nicely, but there's a limit of how much improvement can be had with small, sealed headphones due to their acoustics.

I spent a good bit of time listening to the similar and $100 more expensive Beyerdynamic DT 1350. Though I personally preferred the smoother and more laid-back sound of the DT 1350, I also felt it was the more colored of the two headphones. The Beyer is warm and somewhat muffled sounding when compared to the livelier, but harder sound of the HD 25-1 II, and it's a little more uneven sounding than the Sennheiser, as well. While I think audiophiles would prefer the DT 1350 for music listening, pros and DJs might actually prefer the Sennheiser for its better enunciation and incredible durability.

Fit is important for the best sound and isolation with these headphones, careful adjustment will be rewarded with tighter bass and cleaner highs.

Measurements
This section modified Mar 9, 2012 after new measurements.

Raw frequency response measurements show these headphones seal fairly well and reliably. Bass is slightly rounded and emphasized to 200Hz, then fairly flat to 2kHz. I like this curve for these cans, it gives them a solid bass response even when they aren't quite sealing, and mids that are fully present throughout. The subsequent roll-off to 6kHz is typical and likely heard as fairly flat. The peak at 8kHz is also somewhat typical of headphones generally; I hear this peak in listening as slightly too bright in this area. Highs remain fairly strong after 8kHz, and are about the right level. This frequency response curve is a good result.

30Hz square wave shows a slight bowing as indicated by the bow in frequency response from 10Hz to 300Hz.

300Hz square wave shows slightly more overshoot than I'd like to see; the upward slope thereafter shows a strong upper mids response, which tends to deliver a punchy sound. Both 300Hz square wave and impulse response show some high frequency ringing. I think this may contribute to the grainy sound heard during listening.

THD+noise measurements indicates good power handling and tight bass response. The distortion peak at 5kHz is a bit disturbing and again may explain the somewhat grainy, hard sound at times.

Broadband isolation at -17dBspl is quite good, and with 70Ohm impedance and 48mVrms to reach 90dBspl loudness, these will make good headphones for enthusiasts to use portably, and for pros to use on loud environments.

Summary
The more I listen to the Sennheiser/adidas original HD 25-1 II the more I like them. Not only do these cans deliver a punchy and exciting listening experience that's true to the recording, they're also built to take a beating, and will do a good job keeping the din of the world at bay while you listen to your tunes.

Whether you're an audio professional, DJ, headphone enthusiast, or just a plain old music lover, I highly recommend these headphones.

Resources
1989 Manual for original HD 25 and newer HD 25-1 II and HD-25 SP II.
Headphones are strong in this Sennheiser 2010 financial position press release.
Cool U.K. custom paint jobs on DJ headphones.
A review from the DJ perspective at Scratchworx.
Head-Fi threads here, here, here, here, here, and here.