Amazon is fairly good about returns, as long as you have all of the packaging materials and file for the return within the 30-day window. I have heard murmurings about Amazon cancelling user accounts or assessing fees for "excessive returns" however I have never had an issue with this (I exchanged through probably four pairs of headphones this last holiday season until they sent me one that wasn't defective, and there was no complaint from them - I'm not saying use Amazon as a rental service, but if you return one product for another I highly doubt they're going to sanction you).
Regarding the DAC question:
Digital to Analog converters (which are very simple little chips that appear in a lot of devices you use every day) are more or less transparent/flat with respect to audibility, and this has been the case for years and years. You are very unlikely to be able to pick two of them apart in a properly conducted test, and what is more likely to be at play is either an internally applied EQ curve (this is seen with a lot of DSP powered devices, like SSPs; not something you're likely buying) or different output levels - if you have two devices that are identical aside from output level, we (as humans) will often preference the device with the higher output level as "better" in a comparison.
I absolutely have no respect for the idea of taking a $5 or $10 off-the-shelf part and putting it in a $20 enclosure and claiming it's a $1200 device that "opens up the soundstage, improves the bass, and cleans up the signal" (if you read enough audiophile dribble, it will usually boil down to some permutation of these three things; there's no objective or logical criteria). Those same parts are included in things like your laptop/desktop computer, portable digital devices, televisions, and so on. This does not mean all of those devices are equally well suited to drive all headphones; but that's not a question of the DtoA stage, that's a matter of amplification. Just like you couldn't expect a CD player to drive a pair of speakers (ignore the Sony Qualia that does), you can't always expect it to drive a pair of headphones either. It depends on what the output abilities of the device are (if it's one of those older players with a headphone jack and volume adjustment on the front, exclude it from this discussion) - it's probably going to target a line-level sink, which has a fairly high impedance and doesn't want very much current. That would be something like the "line in" jacks on your stereo receiver.
Headphones are a more reactive load, and need something that can deal with that. Lots of newer devices are built with this in mind, and many can handle the vast majority of headphones on the market (generally only very insensitive or very high impedance models are excluded; and electrostats). Options like the ATH-A900, KH-K1000, and Grado cans will absolutely work plugged into a variety of devices, without any expensive-and-exclusive little black boxes in the mess. If you'd like an amplifier for convenience or to deal with a wider range of headphones, I like the Fiio products - the E9 is an amazing offering, and has a fair price associated with it. It also will take up less space and power than using something like a stereo receiver.
Again, this completely veers away from the "DAC" affair, because it's really of trivial importance. If your "music thing" is able to provide either a suitable headphone output (for the headphones you wish to use; who cares how it would or wouldn't perform with something you aren't using), or a suitable line-level output to a headphone amplifier, there is absolutely no need to spend any more money. Most computers will generally fit into this, however one common problem may be noise from the analog outputs (this has nothing to do with "RFI interference" or any other nonsense, it's the product of sloppy QA and a bad assembly job, and no, you can't fix it - I've heard of people trying to RMA their motherboards and after a few cycles finally getting something that works, but it seems like a lot of hassle). A quick fix here is a soundcard of some sort. Here's about where you'll usually start reading about "USB DACs" (which don't really exist) - these are just soundcards that use USB instead of PCI, PCI Express, or whatever else (anyone else remember MCA or ISA?). They are not magical, and they are not somehow inherently better due to being an "external" component (in many cases they draw power right from the computer; this is one example of how ridiculous audiophile dribble can get - we're moving the device outside to get away from "dirty" power, but we're going to use that same "dirty" power to drive our device, because it's now magically "clean" due to being external (nevermind that it isn't "dirty" to begin with)).
I don't inherently dislike USB devices, they serve a purpose, especially if you're a mobile user with a laptop. There's also quite a few of them that integrate a headphone amplifier and audio controller in one box (like the X-Fi HD External). My gripe is when they cost $1000+ and claim that because they're using USB, they're somehow "improving the sound" in the process; without getting overly technical about it (because there's no need for us to get overly technical) - I'd like to see some objective and hard proof ("I heard an improvement" is NEITHER). And here's where the "nonsense-o-meter" can come out - when you're shown some new magical thing (that costs quite a lot of money) and told that it will improve your entire life, and you ask for some proof, and you're suddenly thrust into an evangelical debate; well, I'd probably say pass on the whole mess. Stick with logic.
And all of that having been said, the final nail in the coffin here is that between any two of these components, which are operating properly and within specifications (we'll ignore all of the wacky and exotic designs that usually measure like hell and are potentially unstable, they usually manage to cost a fortune too (so you can pay to get them, and then pay when the insurance underwriters stop returning your calls because your non-UL certified device caused damage somewhere)), there's not going to be any sort of "sound" to them. They are not tone controls (ignoring the previously mentioned, internally EQd devices). If you want to "increase the bass" or "add more treble" - as the Apple types say - there's an app for that. It's called a signal processor. There's all types - equalizers, compressors, expanders, so-called "exciters" (like the BBE Engine or Creative Crystalizer), and so on. These all do something consistent and measurable (whether you like the outcome is entirely up to you).
Finally, I'll caution you that this understanding is not always popular with all people you'll likely meet. Simply consider their motivations though; some of them want to sell you something, some of them want to be sold something, some of them simply don't know any better, and so on. Just remember that it isn't a value judgment, and that it doesn't have to be a nasty exchange. As I said, stick to logic and you'll probably do alright - when things start getting circular or claim to fundamentally violate at least one of the laws of thermodynamics, I'd start looking elsewhere.
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Okay, so I guess the A900x will stay and I will consider trying the A900. I was unsure if I could return stuff on Amazon so I was concerned for a bit.
Why in regards to the DAC part.
