To elaborate on what Cranky has said, let me point out a basic flaw in the connectivity methodology.
An Integrated amp such as the Norge has pre amp, and a power amp. All inputs are routed through the pre amp. A pre amp expects input from a CD, a tape or other devices, or what is called a line level signal.
The level of a line level signal is defined as a ratio, expressed in decibels, against a standard reference voltage. For our use, Decibel volts (dBV) is used.
The reference voltage for the decibel volt (0 dBV) is 1 VRMS, which is the voltage required to produce 1 milliwatt (mW) of power across a 1 kilohm (kOhm) load. The reference voltage for the decibel unloaded (0 dBu) is the voltage required to produce 1 mW of power across a 600 Ohms load (approximately 0.7746 VRMS).
The most common nominal level for consumer audio equipment is -10 dBV which has an amplitude of approximately 0.316 volts root mean square (VRMS). Professional audio though vary in strength between 1.2 VRMS to 1.5 VRMS.
When you connect a PC or an iPOD to the pre-amp of a integrated amplifier, you are not supplying line level output. What you are supplying is amplified output that can drive headphones or small speakers. This is usually at around 2v or more. Thus this connectivity, by its very nature, is overloading the preamp and will introduce distortion very early. Logically, the output from a PC or iPOD should go to a power amp that would be expecting 1 to 2v input, and can amplify the signal without any distortion.
Unfortunately, as Cranky has said, there is nothing much you can do with integrated amps that do no not have a separate input for power amplification. All you can do is to keep the input to the amp as low as possible so that you introduce least amount of distortion. The ideal levels can only be decided by trial and error.
Yesterday I connected the peramp Front L&R of a Yamaha 663 to the peramp input of a NAD 320. Distortion set in immediately even at 30dB. Fortunately the 320 can be made to act as a pure power amp by removing the bridges in the rear. When I did this, the sound opened up immediately and was distortion free even at 100dB. Of course when you use the 320 as a power amp, you use only the external preamp (in this case the Yamaha 663) for volume control.
Ideally you should use a power amplifier for any signal that is not a line level signal.