@Mr Ee,
Sorry if this is a stupid question.
I remember seeing somewhere that the line input of the WiiM amp is 2V. If the AVR pre-out is 2V, is there any effect of adding a booster?
Yes, potentially there is. It's actually not about the difference in wattage between the Yamaha and the WiiM. This is not important in this case (until reaching the power limits, of course).
It's a problem of the previous "dumb" amplifier's
gain being higher than that of the WiiM Amp. It depends on volume control settings. A pre-amp could.make up for that, even if introducing one more source of distortion.
@craigj265, what amp have you been using previously and how was the volume control usually set?
If you want remotely similar levels from the WiiM and the Yamaha, you will have to set the WiiM to (up to) 100% volume. Switching to one of the WiiM's other inputs like WiFi at 100% might not be a good idea then (remember the warning in the Yamaha manual, brought up.by
@Doublej ). Luckily, the WiiM Amp does allow independent control of volume per source, so it will remember the volume for line-in being 100%, but maybe 25% for WiFi or USB since the latest update of firmware and app. Relying on that alone wilould still make me feel a little uncomfortable.
Did you manage to find the pre-out level setting in the Yamaha's menu? Turning it up to +6 dB will be essential.
I'm sure that none of your devices is defective.