I used a form on the HiFiMan site to return my RE-400 that had a failed cord directly to the manufacturer. Just needed the form and receipt. Replacement is fine so far.
I also sprung for Comply tips, TS-400, large, hoping to improve comfort and seal for bass response. The only stock tips that worked for me were the largest, double-flanged ones, but they were uncomfortable and the seal seemed dodgy. The large Comply tips are larger than I thought they'd be, but seem to work well, without having to jam them far in my ear canal, they just kind of stay tucked in the ear cavity outside the canal (the "external meatus" according to the picture I just looked up). Sound is at least as good as before, the seal for bass response is more reliable, and the tips keep the housings off my ragus and antiragus (who knew the outer ear had so many named parts), a source of discomfort with the originals on one ear particularly. They feel less secure but no problems with them falling out. Kind of wish I'd have gotten the three-size mix rather than large to try the others but the large seem to work fine, and the smaller stock tips didn't seal in my ears at all.
With the new tips, I tried the RE-400 with the iPhone 5 playing 256 kbps files (intended application) and compared that with Asgard 2 fed by Meridian Explorer playing a mix of low and high sample rate sources from my main desktop listening station. No surprise that the RE-400 sounded better with the desktop rig, but it didn't exactly make the bass bloom gloriously. Comparing RE-400 to HD-650 or desktop Airmotiv 4 speakers just isn't a fair fight, the latter two certainly sound better overall with more air, space, and bass, although the RE-400 hold their own in the upper midrange versus these alternatives. That said, I'm not otherwise compelled to complain about the RE-400 bass. For mobile applications, I remain satisfied with RE-400, even after the failed cord, warranty replacement, and 17% deeper investment to upgrade the tips.