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In some cases the difference is blinding, in others only just perceptible once you take into account the fact that just increasing the volume often makes the sound better, and you can get a similar effect to plugging in the E5 just by raising the level from, say, halfway to two-thirds. Where the formula works, you get an increase in volume but also an increase in low-end warmth and clarity, and - unlike with the E3 - you can always switch the added bass on and off.
So where does it work? Well, I've always found the output from the iRiver T7 a little thin and weedy, so it's no surprise to find the E5 working wonders there with most in-ear and over-the-ear headphones tried. Move on to the iPod touch and Samsung YP-P3, however, and things get trickier. I liked the way the E5 added body and some low-end drive to the sound emanating from a pair of Sennheiser PX100s, and there wasn't any negative effect on clarity. Yet with a pair of iGrados, the effects weren't so great: the sound became a little muddy and some background hiss - not obvious, but there - came into play.
I expected similar things from the iGrado's stable mates, the SR60s, but in fact there was another, albeit smaller improvement. This surprised me, because the 32 Ohm SR60s are easily sensitive enough to work from the touch without amplification, but with the E5 in play (and not anywhere near full volume) I could hit my preferred listening level at around the halfway mark on the touch, and the output seemed slightly cleaner and brighter, with a pleasant, rich tone in the mid and upper range. It's not what you might call a night and day difference, but it's there, and I had a similar experience with a pair of Audio Technica ATH-AD500s, with the added bonus that the slightly weak bass was thickened up.
With in-ear models results were mixed. With a set of Sennheiser CX300 Mark IIs there was definitely an improvement in terms of overall body, and I wouldn't put this down entirely to the boost in volume, and the extra power also works with a set of Philips SHE9800s, albeit with lesser effects. However, with a set of Denon AH-C551s - canalphones which already triumph when it comes to warmth and power but fall short through a brash top end and slightly compromised definition - the E5 just makes the sound louder and a bit messier. Be warned: the headphone-level connection between player and amp can add noise, and depending on your headphones and the material you listen to, this can have degrading effects on your overall sound.