Entering the parameters of the PR in WinISD starts with no mass added and the PR's default effect on the tuning frequency of the cabinet. Adding mass lowers the tuning frequency of the cabinet. Adding 9 - 45 gram washers to the APR 15 would tune a 4 cu.ft. box to 21.5 hz. The model is with 500 watts input power and a Hi-pass filter at 18 hz.