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But adding an amplifier is not improving the SQ coming out of the Zip itself. The signal coming out of the Zip is not changing at all. The amplifier is boosting that same signal, to allow use with more demanding headphones. The end result the user hears out of whatever hard-to-drive headphones are in play may be better, which is what you are thinking of, but the inherent sound of the Zip itself remains the same.
I would agree with you both. Technically, there is no sound quality improvement in terms of details, frequency response, etc. The zip sounds very reference for the price. However, if you can't play your headphones loud enough, I suppose you could call that a lack of quality, at least in terms of the range of volume. A quality of a different type. It will definitely sound better to someone who can listen at a proper volume level, rather than straining to hear things. While people with more sensitive phones don't need the extra headroom, therefore they wouldn't benefit at all, nor would they need to.
I've compared the zip to my ipod touch 5g using a BTG line out cable into my C5 amplifier. Sounds 95% identical. That last 5% isn't really a "quality" difference in the "music" sound itself, but rather the noise floor is just a tad more apparent at the loudest volumes. The difference is exactly 7% of the ipod touch volume. In other words, if I volume match the ipod to the zip and turn them both up until they are as loud as they can go, the zip exhibits noise a little more. If I match the ipod/c5 volume to produce the same exact amount of noise as the zip at max volume, I need to turn the ipod down to 93% for the music to be the same volume as the zip music. In other words, if I didn't turn the ipod volume down, but rather turned the C5 down with the ipod maxed and matched volumes, the zip noise would be that much higher.
This is a very small amount, and with any earphones the noise becomes audible near the 90% volume area. The touch has zero noise at max volume straight from the headphone output, but it's also a bit quieter at max volume, so you can't compare that way, because you could match the zip volume by turning it down to the ipod level and the noise would be gone. Basically, what I'm saying is that the ipod line out is a small amount cleaner noise-wise, but this is a very small amount. I only notice the noise with extremely quiet music and the zip maxed with quiet passages. And then it is still extremely low noise.
This doesn't go away with amping. So, headphones only I'd say the zip sounds better. In fact, the zip sounds identical quality wise to the ipod line out (minus the noise difference). The ipod headphone out (5g at least) is NOT the same. It is softer in the details. It is less distinct and energetic sounding. This becomes very clear with certain songs switching between the devices and the line out as well.
Just some impressions...