Just been fiddling some more. With regards to the Surround, it DOES give more presence, agreed, but it's not necessarily better. Horses for courses really. It reminds me of a cheap 80's midi system. Lots of buttons for making the music sounds in-your-face and artificial, but no real talent for portraying an accurate and detailed sonic picture. It's not bad, but it's not brilliant. I'm not sure there's anything wrong with the rear speakers now, but I haven't completely ruled it out.
And, to be fair, there's a whole load of difference between what sort of sound you need when sitting stationary with the engine off, and when blasting along. I'll have to take it on the road. Trauma!
And yes, if you hadn't guessed, I'm one of those obsessive Hi-Fi people