I agree. The stock sound of the E9X left me un-impressed. After 7 years of S54 excitment in my E46, I found my E9X rather boring in the sound department. Until I installed an MPE, K&N filter and BPM tune that is. The S65 then became pure bliss
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For now, the S55 simply does not match it. My M4 makes interesting sounds; but it is more loud and obnoxious rather than sounding highly tuned and refined as the S65.