A passive radiator can be used for tuning a box lower than what would be possible with a port due to size constraints. I haven't studied their potential advantages and disadvantages, but I would expect proponents to say the lack of vent turbulence and the ability to tune deep are its two biggest advantages.AE Speakers makes passive radiators. Nick McKinney seems like a pretty good guy and I think he is very quality conscious. So you might check out his products.
Nick used to spearhead Lambda Acoustics, and AE speakers appears to be pretty much the same thing. I don't know what happened to Lambda Acoustics, but I know they closed several years back and are now doing business as AE Speakers. The technical design principles appear to be very good, using things like shorting rings and cooling plugs and concentrating on cone and spider material for reduced breakup on extended bandwidth drivers.