When it says "MULTICH" it is getting a multichannel PCM signal. It has no idea what the signal was originally (Dolby Digital, TrueHD, DTS, or whatever). Since you still have 5.1, there's nothing more to do. Leave it on MULTICH for the A2. When you add the rear surrounds, you can use Pro Logic IIx to expand it from 5.1 to 7.1.