Well guys, I've been having a root around my new Tourer and have found something rather interesting.
Most aftermarket headunits are around 4x40w @ 10% THD, a rather sizeable amount and perfectly capable of holding your own Bruce Springsteen concert while wafting down the motorway.
I've had my radio/amp module in bits this morning to try and sort out a grounding issue, and decided to find out what the MOSFETs in the amplifier module were capable of. I loaded the datasheet and...
We have just 4x 5.5w @ 10% THD! That'll explain the poor bass and lack of volume.
The BM24 Becker radio / amp module (from several BMW models, available on eBay) is a straight swap on our cars - simply purchase one and replace the amplifier / radio module in the boot (spare wheel well, right hand side) with a BM24 and you'll have 4x 38w straight away. You'll also have twin radio tuners, so a station list will be generated under the current station. You can then scroll through and select any of the pre-detected stations, rather than manually scanning.
Just a heads up before people go fitting aftermarket speakers etc. The speakers are fine - it's the amp that's the let down.
*THD = total harmonic distortion, a figure used to measure amplifier quality at a certain output. 10% is a pretty standard figure, however music will be heavily distorted at this level. THD needs to be below 1% for normal listening with quality.