I am supplying a Hiface USB-I2S module for feeding I2S into the digital input of the AckoDAC. I2S is a lower jitter & more direct digital communication method than SPDIF if implemented correctly. Most DACs do not have external I2S connectors as there is no standard connector. The I2S connection is meant to be kept as short as possible but with good HF cable & connections, a foot or more is probably fine.
Hiface USB-I2S module
This 24/192 asynchronous USB transport using low jitter clocks with optimised power supply feeding I2S directly to a DAC should result in the best computer audio available today.
This Hiface version is focused on I2S with NO SPDIF outputs. It will have PS connectors that can be connected to the supplies of choice; on-board 3.3V regs, batteries or external PS. No power is drawn from the USB cable so no reliance on the often variable quality of computer PS & therefore no need for so called "audiophile" USB cable - a reasonable standard USB cable will be as good as an expensive one.
The AckoDAC is a new line of DACs based on the range of DAC chips from ESS. This was jointly developed by Acko & myself. The first of these DACs is on offer here: http://ackoav.spaces.live.com/