Nothing wrong with using the full range of the dial, I'm not sure why you would want to, or expect to use 1/2 the range to begin with. Arguably it's better to keep the level higher at the source, than to add more gain (or less attenuation?) at the speaker.
I know it's not a direct comparison to working in a DAW with unprocessed tracks, but for playback thru Spotify etc. my LSR 305's are set to +4dBu and 7/10 on the rear volume control, I have a -4dB pad set using EQ APO (
to avoid Windows CAudioLimiter), and I can easily drive the JBLs to audible distortion without maxing out the knob on the M4. Usually I have the M4 at 12-1 o'clock for near-field playback, but at 3 o'clock it's enough to fill my (relatively small) room.
Based on the manuals, the M4 should be capable of overdriving the input of the 305's when set to -10dBv, so I'd leave it at +4dBu unless you actually run out of range on the M4 dial for some reason. Just my 2c.