Flashing Red db on line-in

[f1eng]

I believe that this can be changed on the fly with the remote. Either you will be using ridiculous amounts of gain to get the desired volume or, as in your case, the amp will be clipping. Try using the level meters to get your peaks as near as possible to 0dB.

I would not recommend going anywhere near 0dB. I know most recordings these days are hammering up against the limiter, but they still won't be clipping if properly mastered. There is no limiter software in the Devialet, so -6dB peaks is a good target.
I have been recording music for 50 years now, and whilst with tape a high recording level was needed to keep away from noise (and a bit of tape saturation sounds euphonic) with a 24 bit digital recorder keeping away from 0dB is important.

Sure, perhaps I should have said as high as possible without the display showing orange or red, although I can't see what harm can be caused if you hit 0dB without clipping.

True at 0dB there should be no problem. OTOH unless you have a source of music which you know is the absolutely highest level which will be input into the Devialet there is no way to know how close to clipping you will be.
With 16-bit converters there is not a huge amount of spare dynamic range (though only between music and silence, 16 bit is plenty for most musical dynamic range). With 24-bit converters in the Devialet there is plenty of dynamic range so no need to risk getting clipping.