@smart191, Did any of the above suggestions SOLVE your Luxman L-507ux low volume on balanced inputs issue? I hope they did.
If NONE of the above suggestions work, please post the make and model number of your DAC. As you noted, since there are no volume issue on using your analog inputs, it 'implies' the issue is with your DAC's settings (I think). If anyone has any further suggestions, please add them to this post.
Please keep us updated. Thanks.