In the price ranges the average joe is more comfortable with, there are really no floorstanders that can go convincingly deep enough and powerful enough down to 20 Hz that some muscial recordings contain. Such a capability is reserved to those behemoth and megabuck speakers. When reviewers says so and so budget and mid-price speakers have excellent bass, they're most likely talking about flat responses from 60Hz to 80 Hz - the region where the bass of most pop and rock recordings have. Classical recodings also generally do not contain that much low frequency content, unless there are kettle, timpani bass drums and piano notes in pieces that require low piano keys to be struck (generating 27Hz the lowest on the piano.)
Hence, you're better off with an excellent bookshelf and a sub.
It is always adviseable that the speakers generating low frequencies be separate from that generating the rest (above 100Hz, typically) Using subwoofers is a form of active bi-amping that dviides audio spectral content into specialized amps and speakers, giving the main amps more headroom and power for non-bass greater band-width content and sending the power hungry bass freqs to more powerful but bandwidth-limited sub-woofer amps.
Another excellent reason for having a sub is that in general, stereo or soundstage imaging is best achieved when the speakers are far from boundary walls. You get good imaging but at the expense of bass response. Try it. The farther a floorstander or bookshelf is from the corner or back and side walls, the lesser the bass perceived from your listening chair. Better stereo imaging and lesser midrannge coloration from reflecting adjacent walls. But no bass. The most powerful bass comes from a corner placement. The farther you go away from a corner, the lesser the bass. But you get better imaging and midrange clarity.
Hence, in this regard, an excellent bookshelf speaker is all you need to get excellent stereo imaging and focus. Even budget bookshelfs have excellent mids and high frequency propagation characterisitcs. But you should have a subwoofer to take care of the bass, preferrably in a corner or near wall placement. Using your average joe floorstanders exclusively cannot yield the best imaging and the best bass response achievable. At most, they give a compromise.
The use of subwoofers would have virtually rendered floorstanders technically unnecessary and enters the realm of personal preference. Except for one technical consderation.
That is called low frequency imaging or directionality.
It is generally accepted that the human hear cannot adequately perceive the direction of low frequencies below about 60 Hz. Too bad not all recorded materials contain bass that are non-directional. A kick drum or a bass guitar originating on the right channel do send out bass frequencies that you can localize as coming from the right channel. That's because they generate fundamentals and harmonics above 60 Hz which are directional, and adds to the soundstage illusion.
A subwoofer is really meant to generate frequencies below a non-directional 60 Hz and down well into the infrasonics where the sound is more felt than heard. And you only need one subwoofer for this purpose. However, with most subwoofer crossover filters set at 120Hz to 80Hz, there is still directionality within that range. And summing the left and the right channels' low frequency content into a single sub loses much of low freq directionality and muddles what little is left in the main speakers.
That's why many audiophiles prefer floorstanders plus subwoofer. They prefer a floorstander with a cutover at around 50 Hz so the sub really gives only non-directional bass. And the floorstander will take care of directional bass in the 80-120Hz where it is good at. Another reason is because a floorstadner's bass response while rolled off when posiitoned away from walls, provide a gentler bass roll-off that makes a more seamless and smoother transition with the sub's starting crossover freq compared with a bookshelf that can often sound abruptly bass deficient when posiitoned away from walls.
On the otherhand, using floorstadners and a sub can have the danger of freqiency peaks at the crossover point, if the main speakers do not roll-off fast enough. Solution is to set 2 crossover points - a higher one from the preamp and a lower one at the subwoofer amp.
Using bookshelfs may require the use of left and right subwoofers when set at 80 to 120 Hz crossover. This is done so the sub can handle directional bass in that spectral region.