@Ziggy
Please check the following points:
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When you mentioned that, although it is slightly off topic, it would be very useful to reassign the volume buttons on the remote control to operate a HiFi Rose amplifier when the output is set to Fixed Output, are you currently using an RS520 or RS201?
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If you are using Fixed Output in the Pre-Out Level settings, you can still control the volume by enabling the Software Volume Control function.
Volume control (pre-amp function) is also available through the digital outputs (coaxial / optical).
On the RS151, please go to Input/Output Settings, then click the small gear icon in the Output section. There you will find the Software Volume Control option.
Please enable this feature.
Below is an explanation of Software Volume Control:
Some USB DACs support UAC2 volume control, while others support HID volume control. In this case, when you adjust the volume on the Rose device, the volume command is transmitted to the DAC to control its volume. If the DAC does not support this feature, you can enable Software Volume Control in the USB DAC settings within the I/O Settings menu and then adjust the volume through the Rose app.
Caution 1. Software Volume Control cannot be used in Native DSD or DSD over PCM mode. It is applied only after switching to DSD-to-PCM mode (up to DSD128).
Caution 2. For digital output, MQA Stream is supported only when the volume is set to MAX. If the volume is below MAX, MQA Stream is not supported.