I have Usher S 520's and a Outlaw LFM 2 (8") sub. The Usher's bass starts to roll off around 70hz and goes down to about 50hz. I set my LFM's crossover at about 60-65hz and thus achieve a smooth transition from the Ushers to it for the lower bass. I don't have a "bass gap" iow.
BUT: I usually don't even use a sub with my Usher's unless I'm watching a concert DVD or movie. Because even though the Usher's "only" go down to about 50hz, I find that more than enough. I am using them in a fairly small room, but they in no way have trouble filling it or sounding thin.
And it's good that the Usher didn't try to force the 520's bass lower, because then it would be a "fake" bass that won't sound right and start to mess with the musicality and clarity or the "real" bass. You'd have the lower mid range and upper bass more pronounced to try to make the speaker sound like it has more bass than it really does, and then you will undermine the accuracy of the bass articulation. I'd rather hear the growl of the bass guitar and tight beat of the drums that isn't overpowering but honest than a warm mush of false bass that sounds big and bloated since it's really just overdone lower midrange and upper bass. You can't alter physics iow.
So I'd be wary of any bookshelf speakers that claim to go down below around 50hz, or at least below 40-45hz. They might be playing with specs, or they might be comprimising sound quality in an effort to attract more "bass heads".
My advice: don't worry about how low bookshelfs might go, worry about how well they sound. Then choose the best sounding one (tested yourself or from reviews) and if it doesn't go low enough or produce enough bass for your preferences, then add a sub. This may be more expensive than buying all-in-one bookshelfs that supposedly go from very low frequencies with a 5" or 6" or possibly 8" woofer to very high frequencies with their tweeter, but the system will likely sound much better.
You might also find you are satisfied with just your bookshelfs, I am. If I could go back I wouldn't have spent the money on a sub, since I don't use it much. And I thought, at least, that I was a bit of a basshead. Guess not, I'm a "faithful musical reproduction and balance" head lol.
BUT: I usually don't even use a sub with my Usher's unless I'm watching a concert DVD or movie. Because even though the Usher's "only" go down to about 50hz, I find that more than enough. I am using them in a fairly small room, but they in no way have trouble filling it or sounding thin.
And it's good that the Usher didn't try to force the 520's bass lower, because then it would be a "fake" bass that won't sound right and start to mess with the musicality and clarity or the "real" bass. You'd have the lower mid range and upper bass more pronounced to try to make the speaker sound like it has more bass than it really does, and then you will undermine the accuracy of the bass articulation. I'd rather hear the growl of the bass guitar and tight beat of the drums that isn't overpowering but honest than a warm mush of false bass that sounds big and bloated since it's really just overdone lower midrange and upper bass. You can't alter physics iow.
So I'd be wary of any bookshelf speakers that claim to go down below around 50hz, or at least below 40-45hz. They might be playing with specs, or they might be comprimising sound quality in an effort to attract more "bass heads".
My advice: don't worry about how low bookshelfs might go, worry about how well they sound. Then choose the best sounding one (tested yourself or from reviews) and if it doesn't go low enough or produce enough bass for your preferences, then add a sub. This may be more expensive than buying all-in-one bookshelfs that supposedly go from very low frequencies with a 5" or 6" or possibly 8" woofer to very high frequencies with their tweeter, but the system will likely sound much better.
You might also find you are satisfied with just your bookshelfs, I am. If I could go back I wouldn't have spent the money on a sub, since I don't use it much. And I thought, at least, that I was a bit of a basshead. Guess not, I'm a "faithful musical reproduction and balance" head lol.