0dB at the empeg is referenced against what it considers maximum output. 0dB on its own means not a lot. Was that 4Vpp lineouts? RMS? I don't remember

What is the input maximum for the device? It won't be 4V pp
or RMS.

It's unlikely you'll be able to input a high signal and have a significant negative internal gain as the input circuitry will still need to accept the higher input. So you'll ultimately need to limit or reduce the output of the empeg.

Yes it's best to have the highest output possible, particularly in car audio, but you're not going to hear (or lose) any significantly quality IMO between running the empeg at full and reduce it vs running it at a lower level *unless* the cable between the empeg and the BT device are picking up significant noise.