The standard "Hi Fi" amp is probably 150 watts, total power. The E53 WDS does not list the amp specs.; however it is very similar to the E38 amp, and that amp is listed as having 10 channels at 15 WPC each, for a total of 150 W.

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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