Thanks for the welcome all
As for the audio, I don't think there's anything wrong with it per se (in terms of not working properly), it's just not very good quality. I previously had an E36 BMW 3-series, where I replaced the factory head unit with a Becker unit. The Becker looks totally OEM, but has much better sound quality and a good iPod control interface and Bluetooth phone interface. Add in a pair of amplified 6x9 subwoofers replacing the OEM rear speakers to fill in some bass, and the sound quality was far superior to that of the Bose system in the Z, even retaining the OEM front speakers.
Unfortunately with the Z, it seems far more difficult to upgrade the head unit with something that looks OEM but has better sound quality and retains the steering wheel controls etc. Most of the double-DIN units on the market seem to be quite bling, which I don't really like. Some of the Android-based units look quite good though, although I'm aware that these Chinese products often don't live up to the claims in the adverts!
I'll continue to review previous posts on the audio front, and decide what direction I want to take in due course.