When people say "tuning a subwoofer", they are talking about a ported/venter bass reflex subwoofer.
The vent is tuned to a speific frequency and when this is tuned low like 20hz the gives greater effieceny on the deeper bass region.
The main dissadvantage is that a vented subwoofer will roll of extremly fast (24 db per octave) below the tuning point of the vent.
So this the last thing you want to do is tune too high like 35hz as you will have a huge peak at 35-45 hz almost no output at 20hz.
There are three laws to subwoofers, bass extention (Depth of bass), size od enclosure and power efficentcy. You can pick two but at the sacrifice of one of the others.
Have a large box (2-7 cubic foot in size) ported at around 16-25 hz, use a well built subwoofer driver and use a decent amplfiier between 250 and 750 watts of power.
Examples of this would be SVSubwoofers, HSU VTF, AdireAudio Shiva/ Tempest and Strike AV series.
The SVSubwoofers are excelent examples of a propery designed subwoofer.
The other way is to use a smaller box around 1.5-2 cubic volume, well built driver and use a Heafty amp 750-2000 watts with some EQ, bass boost or Linkwitz Circuit programmed in to make up for the much reduced F3 point. You can also use a clevler servo system to control the cone movement.
Examples are Velodyne CHT, HGS, Paradigm Servo 15. IPL Acoustics SW2/3.
Another method is to use a Infinte baffle this is where you build the multiple large subwoofer drivers into a large space like a attic ajoing room or large cupboard and efectivly greate a huge sealed enclosure. (Generaly size between 4 and 10 times the VAS of the subwoofer driver)
This if done corretly can give huge output right down to lowest depths.