Do I need a subwoofer?

By and large a sub is a good idea.  Most 'full range' speakers are not quite full range.  However, consider that many people run stand mounts happily.  To my way of thinking either get a good sub or just run with what you have.

Two (or more) subs or just one?  There is an overwhelming consensus that multiple subs are better than one.  This obvious truth is unhelpful.  The issue cannot be seen in a vacuum.  First up, there may be space issues.  Second two subs will cost more than one.  I consider it better to get one better quality sub.  On the other hand it may actually be easier to locate two subs than one.  This is on the basis that you cannot get a perfect location for one sub.  Now you really do need to get two subs.

Now, what brand of sub?  I guess reputation and reviews and forum opinions are very helpful.  Let me add a bit more: try to get a sub that will integrate with your system - ie integrate with your amp and your speakers.  Here are my tips:

First, if possible get a sub that takes high-level input.  That is to say it takes the signal from the power amp rather than the pre amp.  (Not a big deal if you have an integrated).  The reason is that your sub and your speakers will share the exact same signal.  This is better because the signal from the pre-amp is a little different than the signal from the power amp.  This is strikingly obvious if you had a solid state pre and a tube power amp.

Second, try to match your sub and the power amp.  If your power amp runs SS with mosfets, then a sub with mosfets would be ideal

Third, try to match your sub with your speakers.  So: paper cone - paper cone; carbon fiber cone - carbon fiber cone (what I have).  Ported design - ported design; sealed cabinet - sealed cabinet.

Best of luck

 I really appreciate the input as I put my audiophile research hat back on.

If you actually do research, as opposed to going with the majority status quo then you will read up on DBA and find that compared to the importance of using 4 subs the choice of which subs to use hardly even matters. The truth is if you stick with one or two there are no one or two ever made that will be as good as any 4 you could buy at random.

Do the research, you will see.

@jerryg123 , that is certainly the easy way out. You will just have to live with more distortion in your main speakers and very little if any bass below 40 Hz and please don't quote specs. 1 meter is a lot different than 12 feet in a residential room.

For many this is the better way to go. Without digital bass management integrating subwoofers can be a real PITA and commercial subwoofers other than Magico's Q subs and perhaps one or two others are pretty bad. I listened to a smallish monitor system last weekend and it was surprisingly good excepting there was no low bass. I can understand why many people would stop there. Those monitors were better than any tower speaker I have heard.

@mijostyn Little salty? I did not quote any specs that was cindyment.

@jerryg123 , You are right in that the volume of a point source system drops of at the cube of the distance at all frequencies. You are also right that a room can increase bass at certain frequencies at various locations depending on the room. However, drivers have a frequency response range, a range where they can effectively radiate the frequencies they are responsible for. What is the low frequency limit of a woofer?  Woofers do not have electronic high pass filters. They will vibrate at 10 Hz even though they are incapable of radiating that frequency usually because they are too small or they can not move far enough. If you stick the microphone right up to the driver say 1/2 inch away you will be able to see 10 Hz on an oscilloscope at some volume. As you move away that volume will drop of rapidly way faster than the cube of the distance.  By a meter you will not be able to measure it at all. As the woofer gets larger (in an appropriate enclosure) and moves farther you will pick up 10 Hz louder until finally the volume is up to the rest of the frequencies at one meter. But at distances farther away the lower bass limit will rise or rather the volume at 10 Hz will fall off faster then the rest of the frequencies. This is why the specification for frequency response is so misleading. Just because a speaker's 3 dB down point is at 28 Hz at 1 meter does not mean it will be 3 dB down relative to the other frequencies at 4 meters. Under most circumstances the 3 dB down point will head north. At 4 meters it might be up at 40 Hz.