Hi,
For the Aux-Out connection, it may not be part of the volume control. The signal may be at a set volume, so when you turn down the amplifier volume, the Aux-Out remains at the same signal level. In this case, you will not be able to use a power amplifier. The manual does not indicate whether Aux-Out is controlled by the volume. Unless you can test this, then maybe a tech support request with Marantz.
For Nord, the amplifiers i refer to are :
Hypex NCore NC252MP twin channel ModuleNord Switchable RCA-XLR input boardAuto sensing 100-240V mains input operation0.5W standby operation650W Main PSU47K Ohms Input Impedance, low output impedanceTHD 0.0015%S/N Ratio 121dB26dB Voltage Gain2 x 200W 8 Ohms2 x 250W 4 Ohms2 x 180W 2 OhmsFully...
www.nordacoustics.co.uk
The above is Stereo 250watts and less than £500, using the Hypex amplifier units which receive favourable reviews - you can search the internet. The power amplifier does not have a volume control.
The Marantz unit is a very good all in one approach. If you move to another solution, what will be your primary listening sources ?
Regards,
Shadders.