Glad to help MPT,
I haven't heard the A8 but there is a small but noticeable difference between the B&Q system in the A5 and the Bose system in the Q7, the sound are similar but differ in that the Bose has a fuller more rounded bass but is ultimately the less accurate for it. The A5 system is surprising spacious, equal to if not better than the Q7 and when you consider the cabin space difference this is amazing. Ultimately the B&Q system is better than the Bose but to maintain an accurate sound balance between the highs and the lows while driving you have to adjust the bass control up a couple or three positions which at normal listening volumes isn't a problem, the higher the volume goes the more distortion is introduced because of the increased bass.
Don't get me wrong, for the money I reckon only one or two OEM systems are it's equal, the Volvo Premier system is one of them and the Lexus IS range system being the other. Firstly the Volvo system, it has the best sound balance for a car of any OEM system around, to include 8" woofers in the front door is a real commitment from a manufacturer to it's sound systems. The only problem I have found with the Volvo/Dynaudio setup is left to right balance, it's centre sound stage is most dependent on where you are sitting, a good system like the one in the Audi still has a similar placement of instruments regardless of where you are sitting. The Lexus probably sits in between that is the Audi and the Volvo, getting closest to the Volvo in sound balance while matching the Audi in placement.
If sound is your number one reason for buying a car then pick the Lexus but all that will say about you is that you are a boring guy who likes his music.