Thanks Rob, I listened to the Beats system in a Beats Polo yesterday and it was bassy and the vocals were distorted and muffled. No treble even with the treble on full. I also listened to the standard 2017 Seat Leon system and it was much better and clearer. I think I'll stick with the regular system as the Beats system might not be as clear and clean as I would like. Besides, I doubt if I'll be able to hear the Arona Beats system (I can't imagine any dealer stocking an Xcellence First Edition) so I would be mad to buy without hearing how it sounds first.
Update - I've auditioned Beats Audio in two new Ibiza's and it's far better than Seat Sound in the Leon. Obviously it wasn't set up right in the 2016 Polo I first heard it in. It's deep and clear in the Ibiza's, just what I want. It's a shame I can't buy a DSG Arona here in the UK with this Beats Audio upgrade (I can add Beats to an Arona DSG in Ireland and all over Europe too - looks like the UK misses out again!).