Ok im trying to tune my two subs together one is a 15 inch velodyne, the other is my DIY model with a fosgate 18 xlc in 4 cuft box with a dayton 1000 hpsa for an amp. 18 is sealed 15 is vented. Isnt there a phasing difference between sealed and vented or group delay ? I also think my box might be a tad big. Ill post dimension later today and pics.