If you only really care about one seating position, then a single subwoofer - carefully positioned within your room and matched with a good pair of speakers - would be what you should be aiming for, regardless of whether that pair of speakers are bookshelf, Towers, or so-called "full range" with their own built-in, self-powered woofers.
The thing you must realize is that the sound that comes directly out of your speakers is not the sound that actually enters your ears - particularly not for the bass. At all times, you are not just hearing the direct sound that goes in a straight line from the speakers to your ears, you are also hearing sounds that first bounced off of a wall or the ceiling or the floor, or bounced between several walls before reaching your ears. It is this ratio of direct sound to reflected sound that allows you to get a very good sense of the size of your room, even with your eyes closed, just by listening.
In the bass, and the deeper the frequencies go, the sound waves themselves are very, very long. An 80Hz note, for example, has a wavelength of about 14 feet, and the wavelengths just keep getting longer the lower you go. 40Hz is about 28 feet long, and 20 Hz is just over 56 feet long!
So think about that for a moment - in the deep bass, the wavelength of the note itself in the air is actually longer than any dimension of your room! In essence, this creates a situation where you are NEVER hearing the direct sound from your speaker or subwoofer. The sound wave itself is quite a bit longer than the distance from the speaker or subwoofer to your seat. So the vibration of the air cannot even form within the direct distance from the speaker or subwoofer to your ear. The first bit of pressure change moves past your ear, bounces off of a wall, and returns the other direction before the pressure changes direction based on the movement of the speaker itself!
Bottom line, when it comes to bass, pretty much all you are ever hearing is reflected sound. And because of that, attempting to play any speaker "full range" almost always results in uneven, inaccurate bass actually reaching your ears.
Your speakers must be positioned where they will produce the best imaging and accuracy for the midrange and treble. That is their primary job. But the best positions for imaging and accuracy in the midrange and treble rarely coincide with the best positions in your room for accurate bass at your seat. So it is almost always easiest and best to use separate subwoofers that can be positioned in the room where they will produce the best bass at the seats. By splitting up the frequencies this way, you can achieve great accuracy throughout the entire audible range of frequencies.
I know there are people who hold onto the notion that playing ALL frequencies from the speakers is somehow more "pure" or closer to some sort of ideal. And, indeed, if you were suspended over 56 feet in the air with truly full range speakers 56 feet away from you with at least 56 feet of free air in every direction around you and another 56 feet of free air beyond the back of every speaker, that really would be the ideal! Even 20Hz bass notes would be directional with that distance between you and the speakers and completely free air with no boundaries all around you and every speaker. But that REALLY is not a realistic listening scenario, is it? It is certainly not even remotely close to your room that you described!
So I would challenge anyone still holding onto the idea of "full range" speakers as being some sort of lofty goal to just play some bass sweeps. For one thing, extremely few speakers can truly remain linear and accurate in their output all the way down to below 20Hz. But even if the speakers themselves could do so, they still would not actually SOUND that way at your seat in anything that could be considered a "real" room. At some bass frequency, the wavelength of the sound waves themselves will exceed the distance from the speaker to your seat, and eventually exceed all of the dimensions of your room. And when they do, you will cease to hear the direct output of the speakers themselves, and instead, only hear reflected sound that is, in essence, omnidirectional and victim to a tremendous amount of sound wave interference due to all of those reflections.
So play a frequency sweep through any speaker in any room, and eventually, its bass response will become uneven at your seat. You COULD move the speaker to different positions in your room until you find a spot where all of the sound wave interference due to all of the reflections does not produce any huge dips or peaks in the frequency sweep at your seat. But when you find that spot, you will almost certainly discover that it is not a particularly good location for also maintaining good imaging and accuracy in the midrange and treble.
So the solution is fairly simple: use a subwoofer. If you care about more than one seating position, use at least two subwoofers and position them so that they will create as even and uniform a distribution of sound waves as possible. For two subwoofers, those positions would be the very mid-point of opposing walls. Diagonally opposite corners will also work fairly well. And if the only available position is, say, on the front wall 4 feet in from the left side wall, then place the second subwoofer on the back wall 4 feet in from the right side wall. In other words, mirror image the subwoofers. Again, the goal is to produce bass sound waves that are as even and uniform in their distribution throughout your room as possible so that every seat gets very similar bass response.
To make a very long reply short: a pair of good bookshelf speakers with a subwoofer can be a superior setup to just a pair of Towers on their own -- even if those Towers have built-in, self-powered woofers. If all you have are the Towers, you have very little freedom in terms of where they can be placed. And if nothing else, both Towers will certainly be at the front of your room, meaning the bass sound wave distribution will always be denser at the front of your room than at the back. That pretty much guarantees you will have some large dips and peaks in the bass frequency response at your seat, and it guarantees that from seat to seat, the bass will not sound uniform -- every seat will hear something different in the bass.
So if getting the Sierra-2 means you will be able to afford a subwoofer, but getting either set of Towers means you will not be able to afford a subwoofer, then I highly recommend getting the Sierra-2. We sometimes get mislead by thinking of subwoofers as some completely separate category. All a subwoofer is doing is turning a 2-way bookshelf into a 3-way full range speaker, or a 3-way Tower into a 4-way full range speaker. It's just that with a subwoofer, you're also able to position that 3rd or 4th speaker driver in a different location within your room than the rest of the speaker!
So keeping all of the drivers in the same enclosure is not more "pure" or more "ideal". It's just more limiting! Subwoofers give you the freedom to work WITH your room, rather than having to fight against it. That makes it a much easier decision, in my opinion
Best of luck in your purchase!
- Rob H.