There were 3 models:
HD25 sp (cheapest)
HD25 [Mark I - but not actually labelled as such]
HD25 MkIIs (I paid about £150 overseas for mine
)
The HD25sp are very warm and rounded: it was my first HD25 pair and it was fantastic compared to anything else on the market I'd tried.
All 3x HD25s can easily drive your iPod to beyond deafening volumes: you'll be harded pressed to find any other headphone which can generate terrific 120+ sound pressures. All 3x HD25s are taut and tight. If you love punchy dance/rock/fast changing soundscape music, the HD25 sp can sound a bit 'blurry' compared to the other more expensive HD25s.
I had the HD25 (mkI) until 2 years ago. It was brilliant and dynamic: perfect for insulating against trolls and goblins chatting away in busy underground trains. The closed back is so so important for the extra detail in soundstaging, and even those audiophiles with their expensive HD800s and porty amp won't be able to listen at such low volumes in crowded spaces without increased aural fatigue.
For a newcomer to the HD25s, you'll notice the difference betwen the SP and the other two models most. On classical music, you'll notice that the HD25 Mk IIs have the strongest instrument separation to an incredible level. I'd never have believed I could pick up so much detail from a crappy iPod, but these are the headphones that make music really sing! That is, unless you are listening to a very poorly recorded MP3 - then the high resolution of the headphones really bring out the limitations in the source material.
This guy did a review of the HD25 (Mark I) headphones and compared them against the Sennheiser HD650s which cost over 3x the price. I'm not surprised at how brilliant the HD25 (lowly Mk I's) were in his opinion. If he was to review the HD25 Mk IIs, they would be even more devastingly brilliant. Quite shocking how well they compete against the audiophile cream
The real test of whether you like them or not, depends on whether you are in busy/noisy environments or not. I am, and find that open backed headphones just annoy everyone and also force me to turn up the volume, which only annoys me and others even more. The accuracy of the HD25 headphones is phenomenal: people complain about the suffocating lack of sound-staging from closed backs, but in my experience, being deafened by loud noise from others and in city streets, it's essential.
