I agree with those that say for music the Low E on the guitar is generally enough. If it rolls off after that then I would not worry about it at all. In fact, unless you have a very high end system, it is better to have bass roll off much earlier (say 60 Hz on a two way) as much of what you get in the extreme LF will all be distortion anyway. Even relatively good subs with large woofers and powerful amplifiers can cause tremendous amounts of distortion and this will ruin good music. 3rd harmonic distortion of a mere 1% on a 20 Hz signal will sound equally as loud as the 20 HZ signal itself (this is because your ear is much more sensitive at 80 Hz than at 20 Hz) You don't see many subs with less than 1% distortion at rated high levels of SPL output at 20hz - so even a sub with a roll off at 30Hz is preferable to all that distortion. Therefore most of what subwoofer and full range speaker owners (flat to 20 hz claims) are hearing on a 20Hz to 30 Hz signal is actually between 40 Hz to 90Hz due to harmonic distortion and the much greater sensitivity of the ear in the 40 to 90 Hz range.
It is also not just a matter of hugely powered subwoofers - unfortunately voice coils get very very hot with all that power and you get significant audio compression as well as the distortion. Here is an example of an excellent subwoofer HT Shack subwoofer test - at 20 Hz it does not get much above a mere 100 db and you need 75 db at that frequency just in order to hear anything at all. This sub does get you an extremely impressive 115 db SPL at 40 Hz (but with 20% distortion however - good for HT but not quite acceptable for music). Furthermore as you get up to 80 Hz the THD remains quite high around 5%. This is still a fantastic sub that defeats most of the competition but I hope you are getting the point about how difficult it is to get a truly musical sub and why it may be better to simply roll off those ultra LF signals!
My advice is to stay away from subs and "full range speakers" (down to 20 Hz) for music unless you can afford the best quality drivers. Good linear drivers are very expensive. This LMS-Ultra driver alone costs more than most complete subwoofers. This is the impressive results in a sealed 100 liter DIY enclosure. Again from HT Shack subwoofer tests. Note the very low distortion at extreme SPL's. Now that is a musical sub!
It is also not just a matter of hugely powered subwoofers - unfortunately voice coils get very very hot with all that power and you get significant audio compression as well as the distortion. Here is an example of an excellent subwoofer HT Shack subwoofer test - at 20 Hz it does not get much above a mere 100 db and you need 75 db at that frequency just in order to hear anything at all. This sub does get you an extremely impressive 115 db SPL at 40 Hz (but with 20% distortion however - good for HT but not quite acceptable for music). Furthermore as you get up to 80 Hz the THD remains quite high around 5%. This is still a fantastic sub that defeats most of the competition but I hope you are getting the point about how difficult it is to get a truly musical sub and why it may be better to simply roll off those ultra LF signals!
My advice is to stay away from subs and "full range speakers" (down to 20 Hz) for music unless you can afford the best quality drivers. Good linear drivers are very expensive. This LMS-Ultra driver alone costs more than most complete subwoofers. This is the impressive results in a sealed 100 liter DIY enclosure. Again from HT Shack subwoofer tests. Note the very low distortion at extreme SPL's. Now that is a musical sub!