Hi, i have finally decided to buy a Seymour Duncan Music Man Replacemnt System, with the Alnico pickup and preamp. But i dont know what the difference is between the SMB-4A and the SMB-4D. As im gonning to be buying one of these i want to know what is different about them and which is suitable for my OLP. Any opinion or comment would be appreciated, thanks. P.S. How does changing the pickguard of your Bass, make the sound and tone any different?