A good 10 inch sub would most likely suffice
You are going to have a hard time reaching below 25Hz with a good 10" driver. It just dosn't have enough displacement to produce notes below that frequency.
What consititutes a good driver really depends on what you are trying to achieve. The bass guitar's bottom note is around 40Hz. The lowest note on the big pipe organs is around 16Hz. Movies put out LFE as low as 10Hz with stuff down to 15Hz being common.
A smaller driver like a 10" will do just fine for most music, but not for some and not for movies.
Bob. Folded horn designs exist in the DIY world as well and are considered to be the lowest distortion designs (assuming a good driver). However, it is going to take a LOT of space to make a horn that will produce sound at 15Hz.
