If the hum is monophonic it's probably from power line. You need to use power conditioner. Try to unplug CD player and bring the volume up. If no hum or just a-little than CD-player is the guilty one. A lot of CD players have digital out but they cannot be dedicated transports for using them with DAC meaning that digital "trash" is corrected inside the CD-player. So first, try to use analogue outs, then again: digital sources are the most volatile devices to malfunctioned or "dirty" power lines and need a power conditioner even more than any amplification ones.
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