There's been quite a few sound system threads with very different opinions and I think it comes down to what music you like and how you like to listen to it. If you listen to the radio or just like music in the background then the standard system is fine. However, if (like me) you love to crank the system up it will quickly sound lacking. The *** has a few extra speakers but (despite the marketing bull) is still lacking a proper external amplifier and therefore isn't going to deliver much punch at volume; everything runs off the head unit, which is limited to about 50 real (i.e non marketing) watts RMS. The sub gets about 20 watts at best, and that's why the bass is awful and gets rolled off at high volumes. The B&O does have an external amp which is rated at about 200 non marketing watts and that will really make a difference as the volume increases.
I have an A6 with the bose system (which has an external amp) and have been pleasantly surprised by it. It's not the best system I've ever heard but pretty good for oem and can get quite loud when you want it to. I've got an S4 on order and ticked the B&O box because I'm hoping it will give a similar experience to the A6 (the claimed power is about the same).
My wife has an A3 with the *** and in my opinion it's poor - it runs out of steam very quickly as you turn it up, sounding thin and scratchy and the bass rolls off noticeably as there just isn't enough power to move the sub at higher volumes. The *** in the A4 is rated at the same power, so I'd expect similar performance. However, my wife thinks her sound system is fine because she only listens to the radio and never gets the volume above 10/30!
In my younger days I installed competition level car audio systems and there are a few important things I remember -
- Number of speakers is irrelevant. Driver quality and positioning make all the difference. Competition winning (based on sound quality, not volume) car audio systems usually have one pair of two or three way speakers in front of the driver plus one or maybe two subs - 8 speakers is plenty!
- Power; you can never have too much! Remember that marketers (including Audi) usually quote peak power, which is meaningless - divide by four to get a more realistic RMS figure. Head unit amps (***) are useless (due to lack of power) - a decent system will always need an external amplifier.
- Garbage in garbage out - a low quality source like radio or Bluetooth will never sound great.
The best sounding car I ever built had 7 speakers, 4 amplifiers (about 1500 real/6000 marketing Watts) and was streets ahead of the bose in my A6. Unfortunately it had no boot so practicality eventually won the day and it had to go! However, it proved the point that decent speakers, proper setup and sufficient power is what's important.
In short, if you like loud or bassy music, buy the B&O, otherwise go standard or ***! Just bear in mind that even the B&O is never going to give you earth shaking bass because it doesn't have vast amounts of power - you'd need an aftermarket sub and amp for that.