As you might expect, much also depends on the headphones. With a pair of Grado SR60s, you gain some benefits in terms of a bigger sound and added presence, but probably not enough to make the expense worthwhile. Switch up to the Grado SR125s, however, and there's a more noticeable impact; even though these are still sensitive, 32Ω cans, you can appreciate the extra warmth, detail and general ‘zing' that you get with the D2 over the stock amplification.
I found an even more impressive boost with a pair of AKG K141 studio headphones. Rated at 55Ω, they're a little harder to drive than the Grados and can, with poor amplification, sound thin. I like them a lot for classical music and - rather oddly - vintage rock, but they're not the most versatile set of ‘phones I have to hand. With the D2, however, they take on a new lease of life. I don't think the D2 colours the sound or boosts the bass in the way that the FiiO E5 does, but there's a new richness of tone in the bass and mid-range with it attached, while clarity - already a strong point of the K141s - is, if anything, enhanced.
However, the D2 also gives you another way to get a little more from your iPod. You see, the iPod's dock connector actually provides a line-out signal, and with the correct cable or adaptor attached, you can take this directly into the D2's input, bypassing the iPod's amplifiers altogether. The result was something of a revelation to me.
I've listened to Massive Attack's Teardrop hundreds of times through the iPod through a bewildering variety of headphones, but I'd never heard it sound this good through the Apple player, the huge bass underpinning layers of detail in the vocals, sounds and samples that I normally struggle to resolve. It's an odd thing. I would plug in the AKGs or Grados directly into the iPod and think "Actually, that's not bad. Maybe I'm imagining the difference." I'd then hook up the D2, play the same tracks and think "Blimey - that's what I've been missing."
The fact is, once you've got used to listening to a decent pair of headphones through the D2, it's very hard to go back. Tracks that used to sound OK suddenly sound thinner, less exuberant and less detailed.
The same holds true when you use the D2 as a combination DAC/Amplifier with a PC or notebook. The difference between the output direct from the headphone socket of my HP laptop and the output of the D2 was a lot larger than I'd anticipated. I'd listen to, for example, the opening strings and weird electronic noises on Joga from Bjork's Homogenic and think they sounded okay. Then I'd plug in the D2 and marvel at the new levels of detail, and the way in which the dynamics of the track grow in complexity and life.
The opening bass line of The Smashing Pumpkins' Ava Adore used to sound perfectly good through the bare headphones output, but having listened to it with the D2 hooked in, it now sounds cluttered and dull. With the Bill Evans Trio's take on Some Day My Prince will Come from Portrait in Jazz, there's a new lightness and spring in the sound that I hadn't heard through the Grados before - or at least not to the same extent.