Can the subs fully develop the 20 HZ signal ?
Yes. Car stereos can have nice clean low bass.
Actually there are advantages to using a smaller room...
Modes are raised higher in frequency in room with smaller dimensions, making them easier to damp.
Cabin gain boosts low frequencies in reverse proportion to room volume - requiring smaller bass drivers and less amplifier power to achieve flat FR and high SPL.
and disadvantages too of course,
Too large (passive xo) woofers in too small room will over power the room, and nothing you can do about it.
The important thing is to size the woofers to complement the size of the room and your desired FR.
Or you can use EQ or active xo to fine tune the bass response to the room volume.
If you were an ant inside the subwoofer would you hear any sound? The internal dimensions are much smaller than any wavelength the sub plays. Of course you'd hear it all, for a short time until you went deaf.