Damian Ratka have found very interesting data about the basic M1A2 hull frontal armor package.
The two interesting sources at DTIC archives, that might answer us what type of front hull special armor the basic M1A2 tank used. These sources are Ballisticians in War and Peace vol.3 and The U.S. Army Laboratories at Watertown, Massachusetts.
First document from Ballisticians in War and Peace vol.3 says to us that BRL (Ballistic Research Laboratory) developed special armor for M1A2 hull front providing 35% more KE and 25% more CE protection vs original M1A1 front hull armor.
The secend document is the U.S. Army Laboratories at Watertown, Massachusetts document, provides us information that Watertown with BRL developed in 1988-1991 timeframe a tandem ceramic armor that provided 33% increase in protection vs KE threats. Coincidence? I don't think so.
So we might safely assume, that while basic M1A2 used Heavy Armor Package with DU alloy in turret, hull used Tandem Ceramic Armor Package.
Sources:
Ballisticians in War and Peace
P. S. Some data about Abrams armor:
ტანკების ჯავშანდაცვა - NATO vs СССР
დასავლეთის ტანკების ჯავშანდაცვა (Abrams M1, Leopard-2, Challenger 1/2 და Leclerc)
ოფიციალური დოკუმენტი M1 Abrams-ის კომპოზიტური ჯავშნის შესახებ
ტიტანის ჯავშანი - ანუ როგორ შევამციროთ Abrams-ის წონა