Your's is probably the same as the E39. The E39 has dual voice coil sub speakers, so each speaker uses 2 channels of input from the DSP amp, insead of single-channel speakers like on the E38.

You could confirm it by looking at the plug on the sub box, or pulling out a speaker and looking at its connections. If the plug into the sub box has 8 wires, or an individual speaker has 4 wires connected, then the X5 subs probably have the dual voice coil speakers.

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input