Hi, this is my first post...
Do integrated CD players use the I2S bus between the transport and DAC? If not, why not? What is the most common connection if not I2S?
It depends on the design. Some CD modules have a DAC chip integrated into the CD Module itself and the simplest players will just use that in which case no interfacing is required. Most standard modules can be programmed to output either I2S or EIAJ and integrated players not using the on-board converter will use one or the other.