There is nothing that will drop in to replace it and have any real effect or sound good, size is the main factor as you don't see many IB/free Air subs below a 10 as you need a rather stiff suspension to control the cone. you could use an 8 designed for sealed (needs a higher QTS above 0.45 to sound best IB) but there are not a ton of really good ones for a free air out there.