Signature Series X5 Rental Help

I have a trip to Washington DC coming up at the end of July, and would like to rent a BMW X5 at Dulles thru Avis for 4 days.

I have searched and can't find any coupons compatible on the Signature series. Does anyone know any coupon/AWD codes or credit card promos that can help me? I have Avis Preferred, Avis Corporate, AAA and many major Credit cards(Amex Platinum, if that has any benefits).

If anyone has any other options for a Luxury SUV at a good price from AVIS or other companies I am possible open to those.

Thanks in advance for your help.