Adding Bass to a signal How can bass/low end be added to a signal?

With a low-pass filtered sub-oscillator running at an octave or two below the signal, there still doesnt seem to be enough bass.

Is there a mathematical function to easily add some warmth or boom to the sub oscillator? Or possibly adding two in unison, slightly detuned?
Let's say the audio is given by $ x $.
Let's define Low Pass Filter by $ h $.  

Whet you need to do is:

$$ y = x + \alpha (h \ast x) $$

Where $ \alpha $ is the amplification level of the Low Pass data.
Once you set which frequencies you want to amplify it is easy to design.