whats the box config.? i know in my truck, 2 18 Fi btls on an AQ3500.1 subs forward port up, it would play down to 20hz no problem.. ( tuned to 34) so, i cut the port down and tuned it to "42hz" ( on paper) and i brought peak up to 29 and also had a peak around 46 or so as well..
subs forward port up seems to work best in my truck ( before i walled it)
subs up port side also seemed to work ok