Passive radiators with floppy suspensions are best.
Passive Radiators (Other Models)
PASSIVE RADIATORS FAQ
Passive Radiator calculator
Rear view of the Thiel CS3.7 10 inch passive radiator.
Passive Radiator Calculation
How to Size a Speaker Enclosure for Passive Radiators | It Still Works
Inside the cabinet of the Thiel CS3.7. Along with all the bracing you can see the 10 inch passive radiator and 10 inch active woofer, as well as a look at part of the crossover off to the right top corner of the picture.
In addition, this Sony speaker features EXTRA BASS as well as dual passive radiators.