Bavsound and all the PnP aftermarket woofers are not subs, they are woofers -midbass and some lower 40hz range. SWS-8Xi are subs -plenty of lower 30Hz range at the cost of upper midbass range. You decide which you like the most -deep feeling bass, or solid midbass.
- Front and center speakers only. Bavsound is PnP and optimized for OEM amp. If you want Focal then get the BMW PnP speaker kit of theirs. Don' know anything about the ES100K speakers.
- Need a 2ch aftermarket amp (100Wrms per channel, JL Audio or Arc Audio) to power any replacement woofers as stated, and add-a-sub PnP harness from www.TechnicPnP.com.
- Everything suggested fits and works with OEM amp.
- Don't need anything else unless you want to replace the whole system. Talk to your local audio shop for prices.
I would suggest replacing the front and center OEM speakers with Bavsound, and adding a 2ch aftermarket amp to power either a pair of SWS8Xi (subwoofers = deep bass) or a pair of Bavsound (woofers = midbass) underseat. Keep the OEM amp. No need to redo the whole system like I did. This will sound fantastic for the price and effort.