There is no doubt that Bluetooth APTX-HD volume on the M6 plays at a louder volume compared to the same volume setting on the M9, and that is with the M9 on high gain. 60 on the M6 is considerably louder than 60 on the M9 Bluetooth wise. Once the volumes are at the same adjusted listening levels there is very little difference in quality. I do suspect Bluetooth implementation is different on the M9 to the M6. The M6 being newer may be slightly improved. Hope it is a software issue and can be resolved. Anyway you are right gLer. But under direct wired IEM connection the M9 is clearly better. Don’t know how the two DAPs compare with Bluetooth headphones or Bluetooth IEMs? Anyone?