i assume you bought these from strathfield?
The GT4s are awesome, way better that the current series of JBL woofers, but just make sure the have lots of power, as they dont seem to make much noise without it, we built a demo board at work that had 2 of the GT4 15s running a GTO6011 mono each, and that thing was LOUD! we had it at the brisbane motor show a couple years back, and it was actually drowning out Northfields alpine demo car that was parked 20m away