Pros - Small, feature-laden, jitter measures LOWER than advertised, sound improvement was significant.
Cons - You can buy the Audiophilleo 2 for almost half the price if you don't need the features and get the same sound quality.
Some years ago I upgraded the clock in a fairly inexpensive DAC experimentally and realised the importance of the quality of the digital input to the quality of the sound output. It wasn't long after that a number of companies came out with digital converters, especially USB ones, since the quality of USB inputs in DACs at the time was generally poor. They also allowed older DACs without USB to be used directly from a computer in a manner that improved over the usually low-quality optical or coax S/PDIF outputs available.
I scratched my head for a long time over whether to buy a Halide Bridge or something from Empirical Audio. While I pondered what to get, Audio-gd made it easy by developing the Digital Interface, making the decision easy. I had, prior to that, used the Audiotrack Prodigy Cube as a converter, but after mine died and I read about their reputation for being un-reliable, I had wanted something else.
When the Prodigy Cube died, preventing me using my Reference 1 DAC, I impulse-bought a CD player, but felt that something was lacking in the sound quality. The idea that converters could sound different hadn't occurred to me., but along with my experiment with clocks, it was to lead me to search out great, and then far greater improvements in the sound quality from my DACs.
As a jump from the Digital Interface, I eventually settled on the Audiophilleo 1. While it was considerably more expensive than the AP2, it has a number of useful features that were helpful, especially when attempting to trouble-shoot a lack of sound coming from my DAC, which has no display indicating what is going on inside. It can also simulate jitter and adjust the output to compensate for longer cables if necessary, as well as including a signal generator, which has been very useful for testing purposes. Part of the expense comes from the addition of a Wireworld USB cable (which is now optional if you want the AP1 but want to save a few dollars). The other thing that appealed to me was that it could be connected directly to my Reference 7 with an included BNC to BNC converter bypassing any possible effect of a cable. The inclusion of an attractive and functional case was welcome too. Choosing a colour was the hardest part.
With my MacBook Pro, connection couldn't be more simple: Plug it in and it works with audio files up to 24 bit and 192 kHz sample rates, including all the common frequencies from CD quality up to that.
With iTunes playback I've not ever had any issues. Using audiophile players, such as Pure Music, Amarra and others I've occasionally had issues of the music not playing. It's important to set the bit rate to 24 bit in Audio Midi set-up so the players can change the sample rate, which they can't do if it is set to 16-bit. Sometimes, after my new MacBook Air is put into sleep mode and woken (in Mountain Lion), I have to restart the AP1 for Amarra to continue working. Back when using Lion, a friend's Pure Music set-up refused to work at all with the AP1, though I've successfully used it, Pure Music I've noticed sometimes needs a bit of fiddling in the audio settings to recognise new components.
With the AP1 connected and set as the main audio output device, the system volume controls the AP1's internal volume, the display of the AP1 updating to show the current volume in decibels, down as far as -72dB. A plug-in infrared remote sensor on a cable is included with the AP1, allowing the use of a remote control after programming to control the volume and some of the AP1's features, which is potentially very useful if you want to use it as a digital volume control.
A cable is also included for a 12V trigger allowing the AP1 to switch on other compatible components in a system when it receives music.
I used the AP1 in three ways: The first by itself connected directly to my MacBook Pro. The second using a Vaunix lab-grade USB hub to power it; and the third using the hub and the Pure Power upgrade.
It was immediately apparent that the AP1 was a significant upgrade over the Digital Interface. While the latter has a smooth and very listenable presentation, it doesn't deliver the clarity that the AP1 does. I did find, using the AP1 directly from my MacBook Pro or from a regular powered hub, that the treble in my system was mildly unpleasant. When I switched to using the Vaunix hub, which is designed for their signal generators and has a high quality power supply built in, any unpleasantness in the treble disappeared, leaving just the clarity.
From both discussions with manufacturers as well as research, my understanding is that despite there being low jitter, the quality of the S/PDIF output is equally important. This might explain why, despite having incredibly low jitter and a very carefully designed internal power supply, the AP1 still was slightly lacking using the power from my MacBook Pro.
This leads me to the Pure Power upgrade, which provides battery power for parts of the AP1 and AP2. In itself the Pure Power (PP from now on) is quite a bit more than just a battery pack. It requires irreversible modifications to the AP1 or AP2, to allow for a second power input. The consequences of this upgrade are harder to gauge, as I was without the AP1 for some weeks. However, after the units had "burned in" (my AP1 was replaced during the upgrade as the USB socket had become loose), I can only describe the result with my Reference 7.1, Metrum Octave and Calyx DAC as the kind of effortless, fatigue-free clarity one expects of the best high-end audio gear.
With the Reference 7.1, the main addition was greater clarity. Some vagueness (something I experienced more with the older Reference 1 which had a more inferior digital input circuit) disappeared, resulting in an awesome balanced of clarity and naturalness. With the AP1 by itself, I'd sometimes wanted to use the Metrum Octave instead, with its more "organic" sound. With the addition of the Pure Power, that desire has gone.
The Calyx DAC is an interesting unit, being USB-powered and having a high-quality USB digital input. The AP1, connected to the S/PDIF input easily bested the USB input. Using both with the Vaunix Hub raised the Calyx DAC closer to the level of my Reference 7.1 I felt (at least with my Stax rig. I think I'd need to try with a high-end speaker rig to tell them apart). Prior to that, the Calyx had, while presenting a wide and detailed soundstage, sounded rather flat and unmusical. Sabre-based DACs already require very high quality digital inputs, due to their high-frequency internal clock, but it was interesting to note that the best possible will bring out the real capabilities of such DACs, which are otherwise at a disadvantage compared to their old-school R2R competitors which I usually prefer.
I'm very pleased with the improvements the AP1 and Purepower makes with my DACs over the Audio-gd Digital Interface (which I was already happy with). The value of the model 1 is decidedly in the features and most people will likely choose the bare-bones AP2 for $300 less. However, I feel that it made a solid $895 upgrade to my DACs, significant because at the level of my DAC large improvements require investing a DAC much more expensive than that.