With many Android smartphones, the achievable maximum volume when outputting the audio signal via line out (with cable) is lower than via Bluetooth (wireless). This is due to the fact that the resistance at the AUX input is set comparatively high to protect the ROCKSTER XS from too high levels (as is possible with CD players). The differences are smaller with iOS devices, since the headphone output is set higher here.
Tip: If you use two devices with their own volume control as a player-player combination (e.g. smartphone and Bluetooth box), the volume can be controlled on both devices. However, if the player is set to a very low volume, the player cannot fully compensate for this. So increase the volume of the player first.