The BMR's do have a bit deeper bass extension than our Sierra-2EXv2, as they should being significantly larger speakers with much more cabinet volume.
While an amplifier should reproduce 20Hz - 20kHz with a ruler flat response (typically within +/- 0.1dB) - I have come across some amplifiers that do boost lower bass signals. Sounds like either your Billie is a bit thin on bass, or that the Superphon might be slight boosted (most likely the case)