Integrating a sub with main speakers has little to do with the bass extension capabilities of the speakers, and more to do with the rooms acoustics. You're getting peaks and dips in response in any room, as high as + or - 18 dB in many cases. This is what causes subwoofers to be "boomy" or sound uninspiring more than anything IMO...a small range of frequencies is 9 -18 dB louder than the reference level bass frequencies, and could be 20 or more dB louder than the dips caused by null points in the room...so when you're setting your subs level, you either have some frequencies way too high, or way too low... I have a + 12 dB peak in my 20 x 20 ish room around 28 Hz or so...which is around the transfer function frequencies of my room, then there's two 8 -12 dB dips that generate a broad dip starting at 40 and going to 65 Hz approximately exactly in the range my main speakers would ideally be crossed over to a subwoofer at.
I have a few bass traps that help, but ultimately it comes down to aggressive parametric EQ'ing to make things right.
I believe a room's acoustics are the largest factor leading many 2-channel stereo purists to a pre-mature conclusion that subs don't work with speakers for music.