Same thing happened to me (after I became a member). I called B&N and they said that the discount is off of the list price, not the internet or member price (explains your Bach situation). Also, if the member price results in a steeper disscount off of list than the coupon, then the member price applies (which is why if you try the coupon with the Beethoven set, the coupon is rejected. The internet price is 20% off list and with the 10% member discount, that totals 30% -- more than the 25% coupon discount).
Sneaky. Still less expensive than buying them at Amazon, even with CA sales tax.
David
FYI, for those that are not members yet, there is a one time 25% discount that can be used as a new member. Then you can use this discount (per the rules above) to buy another boxed set.