Sub vs. three way

My bookshelfs are 6 inches and play down to about 80 Hz. Is adding a 10 to 12 inch subwoofer essentially the same as having three way speakers?
No not at all. I think everyone would love a pair of speakers that go all the down to 20hz but those speaker are very very expensive. Adding a sub to monitors will give you fuller sound but bass at 20 to 80hz will be very directional. My monitors go to 70 hz (50hz at -6db) and adding a rel b3 made a world of difference (all for the better for me). Now I would like to add a second sub but most likely never will. On the other hand if you can center your sub inbetween your monitors you wont really notice the bass being as directional.
No not all the same. Essentially a three way speaker will not use the same design compromises as a small two way. A dedicated midrange should allow much lower distortion at higher playback levels and if done correctly can achieve much better dispersion than a two way (two ways tend to beam because they use a large 6 inch woofer right up into the midrange). Beaming means that the off axis response is out of balance with the on axis response and makes the speaker ill-suited for far field listening (where you hear a lot of reflected energy). Two ways are best when used in near-field configurations. Three ways are best when used far-field (they actually work in near-field too but are so big that it can be cumbersome to get them positioned appropriately for near-field)

In addition to the above problem, if you add a monster sub to help two way near-fields work in a big room then you will also run into compression issues with the midrange and tweeter - these will sound harsh as the small two way was never designed to play really loud (as it needs to do in a far-field application).
Shadorne hit the nail there. It all depends on the room. If your room is huge I would say go for three way with large drivers as opposed to small monitors and a sub. If your room is small, monitors with a sub is the way to go. I have monitors and subwoofer configuration in smallish room of 11'x17'x8'. The 4" drivers of my speakers are rated at 35Hz but the bass roll-off at around 60Hz in my room. The subwoofer really helps in fleshing out the bass and just like Xti16, this made a huge improvement to the sound of my system.

If your room is to the smaller side of things, I would highly recommend that you add a subwoofer to reinforce the bass of your system, more so since your monitors can only manage 80Hz. In reality they would roll-off at a higher frequency due to placement from wall boundaries. The quality of the subwoofer plays a part too as inferior subs with large drivers may not perform as well as high-quality subs with smaller drivers.
My room is 12 X 24. I'm considering what effect the long shape will have.
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BTW, bass from a subwoofer is no more directional than from full range speakers, i.e., it is not.
Sorry Bob I have to strongly disagree. My sub is approx 1 ft to the left of my left monitor and the deep bass is louder from the left side of the room. To me that is what I am calling directional. I am also sitting in the 'sweet spot' when I hear this. I had the sub on the right of my right speaker and the deep bass was louder from the right. My room dimensions are approx the same but I'm doing a 7ft equilateral triangle.
I wish I could center the sub but I cant with my setup
bob is correct, unless you put a sub in an overly resonant spot like in a corner in a small space, the sound is not directional. your directionality sounds like a function of room treatment, placement, standing waves, and yes in that set up the bass would seem 'off center'. what bob pointed out is a proven fact of accoustical science. the long room will be a benefit for the longer deep bass sound waves to develop before reflection interferes. and i also take note of statement that 2 ways are best in nearfield and 3 ways in far field. that is much too broad of a statement imo. i use 2 ways and a sub and have zero issues with directionality, fwiw, i have treated corners and walls. go for a good sub and just take your time tweaking it. good luck :3)
All of your current examples, are not quite accurate.
I have been able, to have, a sub, run by a separate amp, that
has been tweaked, to emulate a first order crossover,
while maintaining, time and phase, coherency, of the complete
speaker system!
fwiw, i have have phase time coherent 2 ways and a self powered sub x'd over at 35 hz. my speakers get full bandwidth signal and are not x'd over for sub extension. i do not get any off center sound problems. as sound from my setup from sub is so low i cannot imagine that phase and time coherency would play any role as most of that energy is felt or secondary harmonics. with tube amp the sound is seemless in this regard with two ways and their first order x-over design. my point here is that really deep bass given breathing room and good placement is not going to offer directional issues and the poster should go for the sub and not worry about buying 3 ways or any other main speakers for that matter. a sub will be fine.