Whether a piece of audio has it's power transformer located outside it's chassis, as is the case of the ICBM and it's wall-wart power supply, or internally, anything that gives the power transformer a cleaner sine wave will make it easier for the piece of equipment to generate clean DC voltage(s) for the audio circuitry.
In the case of linear power supplies (such as the ICBM's), there are waveforms other than pure sine waves that give the rectifier diodes, filter capacitors and regulators an "easier time".
In the case of the ICBM, the wall-wart takes in (about) 120vAC 60Hz from the wall outlet and gives out (about) 16vAC 60Hz to the ICBM chassis where the ICMB rectifies, filters, regulates, and filters again to get the DC voltag(s) needed by the active cirtuitry inside the ICBM.
The PSAudio P300 and the ICBM will work fine together. The type of powersupply that the ICBM has is the type that the P300 works really well with. Beings I don't work for PSAudio, I can't claim that the P300 was designed for the ICBM's type of power supply, but if it wasn't, I'd be suprised.