Originally Posted by Bond 007
+1 A sub does nothing for mid bass.
That's not true at all.
Let's assume "mid bass" here is upper-bass: say 150hz-250hz (cause if it's below that: the sub may actually be the source depending on crossover).
Now I want you to imagine the LF driver of the bookshelf trying to put out a pair of waves. One is 90Hz, and the other is 200Hz.
Remember that the distance traveled to make a given noise increases as the frequency drops. So here's this little speaker pushing itself near XMax to get this 90Hz sound out with good volume and, while sitting at one of those peaks, the 200Hz signal also peaks. You peg x-max and get physical clipping. Not good for sound quality.
Now let's imagine that we've moved that 90Hz signal off the bookshelf entirely and asked our subwoofer to do it. Now that driver is sitting with nothing to do except the 200Hz signal. It can do much more moving for that signal than it could before (more SPL) and is no longer clipping.
Better "mid bass" thanks to a subwoofer.
You can get some awesome bookshelf speakers that put most towers to shame if you can afford it. This doesn't mean that you can deny physics. A 2" driver, no matter what material and engineering will produce bass like a 15". But you can get some bookshelfs with a 6" driver and a good sub and get better sound than many towers. That being said there are advantages to multiple drivers also. Having say 3 6" vs 1 6". That's why usually the best speakers are 3 way. But in order to take advantage of that you have to have an excellent crossover system which is what most speakers lack.
Again, it depends on what's it worth to ya.
It's not just that though. Let's assume that our speakers must perform from 80Hz to 20,000Hz. That's about 8 octaves. Let's assume that the load is split evenly among the drivers.
That means each speaker in a 2-way design is primarily responsible for 4 octaves of sound. Each driver in a 3-way is primarily responsible for less than 3 octaves. It becomes much like a discussion of uni-driver speakers (except the degrees of difference are lower).
This midrange on a bookshelf must go lower than the midrange on a 3-way because its crossover point is lower (it has to reach the sub). As a result: it's often larger, and therefore has to start lower, which means that the tweeter is also being asked to go lower... and remember what I said earlier about LF signals interfering with sound reproduction at higher frequencies? That still applies.