You need IRON not air for audio (low) frequencies.
Audio frequencies don't have the magic of Radio (high) frequencies.
They need a magnetic medium to travel thru'
If you really have to use a mod tranny, then you will have to wind your own and it's going to need lots of turns so get a decent low end response.
Something like a T400-26D with min 150T on the secondary.
However as you'll be running a 12V carrier, the mod tranny will double this (nearly as there are losses) to 24V which is dangerously close to the 25V max CEO of your devices.
You'd have to run the amp from 6V to stay safe.