My general thoughts on the ‘how low do I need to go’ issue:

It is my understanding that on the majority of commercially available music recordings a low-frequency roll-off has been introduced. Strictly speaking, you would not need the ability to reproduce below 20Hz, or even 25Hz, unless you are concerned about the following:

- You listen to music in which, for the sake of adding a room or vehicle ‘shake’ factor, the artist/mixer has added a lower octave that has been synthesized or artificially added based on existing fundamental frequencies in the music. Depending on the style of music, the amplitude of these enhancements can be fairly large. Unless you’ve heard this as part of a particular recording somewhere else, you wouldn’t miss them.

- You listen to music ‘recorded live’ in a large venue with very low frequency reverberations that have, in a rare instance, not been rolled-off. These frequencies are very low amplitude but add to the sense of realism rather than being strictly a part of the music being performed.

- Most feature film mixes rarely contain information below 40Hz. Occasionally a feature film mix will have both musical and effects low frequencies occurring simultaneously. If one of those frequencies is a musical 55Hz and another is an effect of frequencies warbling around 40Hz and these two sources happen to be of roughly equal amplitude, significant sideband frequencies are going to be created at the sums and differences of 55Hz and 40Hz. One of those would be 15Hz. I’m not advocating buying a subwoofer good to 10Hz or something like that, but if there is still some output, albeit 3dB to 12dB down, into the frequency teens, a portion of the lower sideband will be heard as the two higher fundamental frequencies mix. In this case the amount of the lower sideband that you would hear is determined not just by the point at which your subwoofer’s low end is rated, but also how fast lower frequencies effectively roll off below that point.

A related matter: does your listening environment significantly suppress or exaggerate very low frequencies? Saving a bit on the particular subwoofer model and spending the difference on acoustic room treatment may be better than just buying a lower-going sub.

In any case, if within your budget, you might spend just a bit more to gain lower frequencies, it is probably worth it. If you have to spend two, three, or four times as much to go from 25Hz to 18Hz or 20Hz down to 15Hz, unless you have the extra money to spend without sweating and it is very, very important to you, that much extra money may not be worth it.