I have a set of the backbox speakers in my LTBR. I couldn't notice a difference between them and the stock ones. I moved them to my XMLE and cannot tell the difference there either. For Sterns, only the Woofer is worth upgrading.

Well, maybe you can tell the difference, I couldn't, but I am not an audiophile. For me, all the sound benefit came from the sub. Now, I only tested in my ACDC and XMLE because it was easy, just swapped the entire DMD/speaker panel. Not sure if there is a difference in the sound cards or programming between these two games and the older ones such as Tron and SM? At any rate, I love the sub in the Sterns and the whole shebang in WPCs.

Are you talking about the entire setup or just the backbox speakers alone? Of course the sub makes a difference. How many of you have actually only installed just the backbox speakers to compare? I doubt very many of you have. How many have only installed the backbox speakers in ACDC? I bet fewer still.
Also the game's system makes a difference too. The OP asked specifically about backbox speakers in ACDC. Talking about how the entire kit sounds in any other system is like comparing apples to cucumbers.