Okay, before getting into the details of the comparisons, I really need to go over all the updates and upgrades to the computer, other system gear, and listening methods.
First of all, most of the experts in the industry have already figured out that the Mac Mini has been providing the best over all performance from A/B listening test as a source. Even comparing it to Mac laptops, etc, it still comes out on top. There are several reasons for its rise to the top but I don't see need to go into those now.
Upgrades made to the Mac Mini were also made that brought it up several levels in performance and I must be up front about what was done. I wouldn't want anyone to think that they will get the same results that we got if using a lessor quality computer.
The Mac Mini that we used had a solid state hard drive. That takes it up a level. It also had all of the music stored on an external hard drive. That takes it up another level. Then it has all other operating systems running in the back ground completely shut down. This makes a surprising difference and again takes things up another level. This particular Mac also has been upgraded to 4 megs of ram. Also, an adapter is used to convert the input plug on the power supply to an IEC so an upgraded power cable can be used. We were using an Electra Cable B-7 PC that was further enhanced by plugging it to an Uber Buss. When all of this is added up the result is really extensive.
Next is the USB cable. I have tried a lot of them including some in the $450 to $600 range. The db Audio Labs cable is the best I have used and very neutral sounding. It was used with all DAC's.
All DAC's were well burned in with the least amount on time being on the db Audio Labs Tranquility DAC. It was a stock unit with the standard Mundorf caps.
Prior to the comparison, extensive listening was done with various power cables and conditioning. I run my personal DAC with an Electra Cable power cable into my Dodd Audio balanced power supply that is plugged into an Uber Buss. This really drops the noise floor and improves the sound from top to bottom. Clearly all the DAC's came up a lot in performance when plugged in with this top level conditioning system. But we decided that the best and most fair thing to do for the comparisons was to plug each DAC into the wall using a low cost and commonly available power cable. We had two identical PS Audio power cables (two were needed for the PS Audio DAC and transport) that we used. They were the AC-10 model.
See here:
http://www.psaudio.com/ps/products/detail/perfectwave-ac-10-power-cable?cat=cables-accessoriesThe rest of the system was an off the grid Dodd Audio buffer driving the big Dodd Audio, mono block, tube amps. Speakers were Super-V's totally decked out. The listening room was also fairly well optimized and treated as needed. The system easily revealed in detail each subtle difference between any change of any kind.

In listening tests we used various tracks and often swapped places taking turns from various listening positions so that everyone got a good feel for imaging and sound stage cues.
We later also made some comparisons using the Dodd Audio balanced power supply and Uber Buss to see what effects that had on various DAC's and how much benefit it gave to each one.
We covered the spectrum in price ranges with the following DAC's:
HRT Music Streamer II at $149:
http://www.musicdirect.com/product/87045The Onix DAC provided by Skiing Ninja at $629:
http://www.skiingninja.com/DAC25-Upsampling-DAC-p/onixdac25.htmdb Audio Labs at $1,495 ($1,295 for AC members right now):
http://www.dbaudiolabs.com/The Ayre DAC at $2,500:
http://www.ayre.com/products_detail.cfm?productid=18And the PS Audio transport and DAC at $6,000 for the pair:
http://www.psaudio.com/ps/products/list/audio/