The iD100 uses a system that varies the SPDIF output frequency in 2ppm (parts per million) steps to keep the output audio in sync with the incoming USB audio from the Apple device docked with it.
This provides the best possible audio performance. The vast majority of DACs on the market today are able to respond to this alteration in the signal without incident but we are aware of some designs which are not sympathetic to this carrier variation.
We are aware of two designs in particular that we do not recommend for use with the iD100. These are the Audiolab M-DAC and the Wyred 4 Sound DAC2.
Please note that this is not a reflection on the audio performance of these DACs, simply that they are not an ideal partner for the iD100. Partnering these DACs with the iD100 will not cause damage to either piece of equipment but may result in intermittent audio dropouts.