Just came across your query regarding the "p audio" so thought to help you for it.
If can have try to "P Audio GST18-1500" which can be surely alternative for your "audio sn4-60f".
for your better understanding I am mentioning brief information about them.
"The P Audio GST18-1500 is an 18 inch (457mm) diameter high performance very low frequency sub woofer transducer. The design utilizes a very large format 5 inch (127mm) nominal voice coil diameter with a very high power copper based coil configured in an inside/outside wind geometry. The large radiating area and deep bass performance insure that the GST18-1500 is capable of very high impact low frequency response.
The GST18-1500 also features a very long air gap height that improves transducer linearity.
The design features very large diameter silicone impregnated spiders to insure long term linearity. The GST18-1500 also incorporates a proven double spider design that effectively controls off axis displacement of the coil assembly."
We do sell our product all over the world.
To know more about it do visit us.
And if you have any query than kindly message me.