I have done this and I believe that when I picked up my tickets I showed my DDE card and got the discount then. I don't recall being able to get the DDE discount at the time of reserving tickets over the phone.
It has been over a year. It could be I was charged the correct discounted amount and had to show the DDE card when I picked up my tickets.
I think the AP discount is only on the last shows. But really don't recall the DDE.