I honestly don't know about in the e46, but in the previous model 5-series, the 'subwoofers' in the premium/DSP sound system are a bit on the interesting side.
Nokia made and branded 4" dual voice coil longthrow drivers in a deeply channeled high-density plastic tray, also Nokia labeled.
It wouldnt suprise me in the slightest if Nokia made the e46 speakers.