There's an interview with Andy Mackay today over at SDE. He had this to say about the 5.1 mix:
The article also ends with the following:
If the record company really wanted to boost sales... all they would have to do is re-issue Avalon...they wouldn't have to remix or remaster the surround version..of course a lot of us on here would have to be talked off the ledge of a tall building...considering what we paid for that SACD...I know a lot of fans have never heard the surround version...due to the cost..