After the stock earbuds on my 4g iPod broke I was determined to replace them with an aftermarket set that was supposed to have better SQ for the $. Budget was under $50. Here's what I tried:
Sony MDR-EX71 - These are the popular earbuds that you always see reviewed on iLounge, with many users claiming superior SQ over the iPod earbuds. This just wasn't the case for me. It had better bass yes, but almost NO mids. Highs were nice and not overbearing but I need that open-air sound for a somewhat wide soundstange and these Sony's just didn't provide it. I can't believe people actually think these sound better clarity-wise.
I decided to keep them anyway because I use them sparingly at work and didn't want to go through the hassle of boxing them back up. After 93 days they broke, right when Sony's warranty ended.

So back to the drawing board...
Creative EP-630 - These got great reviews all over Head-Fi and was $28 w/ free shipping at Amazon so I went for it. After 10 minutes of listening I put them back in their case and plan to ship them back to Amazon for a refund. Horrible sound with a very tinny sound. Again better bass than the iPod buds but that was it.
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So by now I'm frustrated. I hate buying things on the net and being dissatisfied. I just decided to go with what I knew was good and re-buy a pair of the stock iPod earbuds. I went to the Apple store and saw there is a new model, seems they were updated along with the new Nano.
Model #
MA662G/A
Not only do these sound better all around but they are more comfortable with rubber edges. Sleeker, more durable design too. Treble doesn't seem to be as good as it should be, so these aren't perfect, but for $30 I doubt you can find a better earbud.
So if you're looking for a replacement for your older earbuds look at the updated model first.