OK here's what you need to do to use the WiiM as a preamp with your gear. The 740A does not have the ability to bypass its internal preamp and be used only as a power amplifier so you will still be passing the WiiM output through the 740A preamp.
- Set the volume level on the 740A to approximately 25%.
- In the WiiM App, turn off the fixed volume level on the WiiM.
- In the WiiM App, turn the volume level all the way down.
- Turn off or unplug your WIiM.
- Connect your Marantz CD player to the SPDIF IN of your WiiM using the WiiM supplied digital optical cable.
- Connect the WiiM LINE OUT to one of the inputs (1-5) on your Cambridge Audio amplifier using the WiiM supplied RCA cable.
- In the WiiM App, under the Device then gear icon, set the Audio Output in the WiiM device settings to Line Out.
- On the Marantz CD player, change the output to digital. https://manuals.marantz.com/CD5005/EU/EN/YRWWSYloumwjaq.php#GGADMLofpisxzc
You are now ready to test your setup. The easiest way to do this is to select an Internet stream on the WiiM. Once you have done this and it appears to be playing (look at the WiiM app) you can slowly raise the WiiM volume. If you are about halfway on the WiiM volume and it is not loud enough for you then you should raise the volume on the 740A a bit.
Once you have confirmed that your setup is working, you can confirm the CD player connection. Pop a CD into the Marantz. Hit play on the Marantz. In the WiiM App, select Optical Input to select the CD player. You should hear the CD through your speakers.