Sounds like there is something seriously wrong with the design and tuning of your box.......Those 18s ought to be all over that 15".............Of course the types of vehicles you guys have plays a huge role as well...........If his 15" is in a CRX....lol......and your 18s are in a Suburban or Expedition that's not exactly a level playing field......get us some dimensions on your enclosure/port.............