Treatments are as follows, either bass traps that go in corners of rooms, which includes floor/wall and ceiling wall, if done correctly. However, most people start with the front and rear corners of a room which make a huge difference to your frequency graphs. Bass collects in corners of rooms, by addressing this you will flatten out the overall frequency to suprising results.

Secondly, you have FRZ's (first reflection zones) which are placed on the side walls and ceilings to tame the first reflection to the listener. Otherwise, the sound that arrives, direct and indirect, is skewed at your ears. To much treating is not good, but the above added to normal room furniture makes a huge difference.

I have never read about treating behind a false wall, at least on the reputable websites that know about this topic. I can't see why it would be worth your time.

Finally, using multiple subs in your room AND treating your room properly will result in a great outcome.
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