Which DAC is better?

I think all 3 choices are very good. Ayre happens to be my favourite in solid state electronics. Naturally, that would be my first choice. I've also had Arcam CD players. Both products sound great, but they are very different from each other. Arcam digital products are very listenable. They have a refined, polite sound that you can listen to all day. The Ayre is completely different. The sound is very liquid and pure. It has tons of resolution, as well. Some say it is analytical but I would disagree with that. For all its resolution, it is a very musical component. Given the vast difference in sound between them, I would strongly urge you to listen to both before making a decision. I am not that familiar with the Sim but if I had to guess, it would fall somewhere between the other 2.

I own the Simaudio 100D, the little brother of the 300D, and it's a wonderful, natural sounding DAC.

If your primary source will be USB, though, the 300D is probably not the best choice. Simaudio makes it very clear in their description of both the 100D and the 300D that they put their effort into the SPDIF circuits and include the USB input mostly as a convenience.

Yea, the conversion to a computer-based source can be daunting at first, but if you're inclined to keep at it, it will be old hat in no time.

My first-hand experience with different DACs is somewhat limited. Have an Ayre and an MHDT Havana. They are very different animals. The MHDT is a non-oversampling (“NOS”) DAC. That is, it uses cost no object DAC chips from, say, 20 years ago with a USB (and other) inputs. If all of your source material is standard CD (the standard is called “redbook” and is encoded with a 16 bit word length and sampled 44 thousand times per second, which is what 16/44 represents) then a NOS DAC makes some sense. In effect, it’s chips address all redbook material in native resolution because, you guessed it, the best chips from the hey-day of CD ran only 16/44. (Or, at least that’s my imperfect understanding – the Ayre doesn’t “upsample” either, but will run high-rez up to 24/192, while the Havana will not…). But if you want to run any higher-rez material or upsample, one of the DACs running these old-school chips isn’t your answer. Personally, prefer the Ayre, but there are those who don’t. Also liked the Bel Canto, which is actually not a USB DAC, although it does have multiple inputs and a volume control, so can function as a stand-alone preamp as well. To run USB into the Bel Canto, you need a separate Bel Canto converter ($500). Sounds great, and a fine non-asynchronous USB solution. I went with the Ayre.

Otherwise, the original (pioneer, really, I believe) of asynchronous USB is Wavelength Audio. They do multiple iterations of tubed DACs that are meant to be very nice. For years, Wavelength was the only game in town for asynchronous, and everyone else was either working on different technological answers to jitter in an off board DAC, decrying USB DACs as inferior, or waiting it out. Ayre, for it’s part, licensed Wavelength’s asynchronous tech, and thus became the first (and for a spell, I believe only) solid state asynchronous USB DAC on the market. Think it was these two that really opened up the market, and since then there has been a real explosion of asynchronous USB DACs (presumably using iterations of the basic technology to do the same thing). These days, seems that asynchronous is the prevailing way to go with USB to a DAC, but there are many that offer different solutions as well (i.e., the Bel Canto or the Simaudio). Everyone, from “The Names” to newcomers seems to be getting in on the act, so there are lots of options.

As for what’s caught my eye – and admitting that I have no first-hand experience – been interested in the offerings from the likes of Naim, ARC DAC 8, and recently seen some good feedback on the Rotel. The Benchmark DAC-1 is another, and one that seems to be very polarizing (it is from an established pro audio shop that has made a big crossover, and folks seem either to love it or hate it). More recent-ish additions, the offering from Empirical Audio (which is very active on these forums) looks interesting, and a lot of good feedback regarding the Wyred4Sound DACs. I’d love to hear all of these, but haven’t had the chance. So, lots and lots (and many more) to choose from. Best of luck and enjoy.

The new Cambridge Audio Dacmagic "Plus"...the USB works swimmingly...a friend has one, I heard it, it's sweet...my original Dacmagic has a sucky USB system that I don't have a use for so that's fine.

I think by the word super, you meant "asynchronous"

Not all USB interfaces are designed alike. Most are synchronous, with the PC acting as the master clock. As you can imagine, the PC clocks are not the most ideal for for high end digital audio

Asynchronous creates a master clock outside and puts the PC as a slave for sending data. It results in less jitter. Most of the better async USB DACs tout this feature.

And if that's not confusing, enough, not all asynchronous USB interfaces are identical either.

I've had very good results with a Wavelength Wavelink USB interface vs an M2Tech Evo, despite both claiming an async interface.

One of the best USB interfaces I have used is the OffRamp.