A single 1723 Subwoofer 1S vs a single 1723 Subwoofer 2S is an easy answer, the bigger one is better. It has dual drivers in a push-push configuration that cancel out any side-way force and it couples better to the room as well. Obviously, it has more headroom and output with dual drivers and a 400W more powerful amplifier.
Dual 1723 Subwoofer 1S vs a single 1723 Subwoofer 2S on the other hand is a much more interesting question, and there is no straight answer to which solution is better. It depends on some factors that we will look closer at here.
Firstly, if this will be used purely in a music setup, then we would in almost any case recommend dual 1723 Subwoofer 1S. The reason being that you are able to place each of the subwoofers close to each speaker and that ensures the best and easiest way of getting the properly integrated so that all the bass sounds like its coming from your speakers.
However, if you don’t have enough space for them, and you are able to place a single 1723 Subwoofer 2S closer to the middle of the front stage in between the speakers, then this will be a solution that also couples very well to the speakers.
For movies or mixed setups, the general suggestions above still apply, however you should take into consideration that placement in a corner will help the subwoofer have a lot more output in the deep bass compared to having it in the middle of the wall. Another caveat with using dual 1723 Subwoofer 1S with them placed with a distance apart is that they will not acoustically couple on the entire frequency range meaning you may only get a 3dB added output from the second subwoofer above a certain frequency point, instead of a 6dB output that would be achieved by stacking them on top of each other. In that sense you get more output for the money going with the larger 1723 Subwoofer 2S.
With achieving the smoothest possible frequency point in mind, the setup with dual subwoofers is hard to beat as you can move them around the room to get the best integration possible. Although it may affect maximum SPL this will be the best way to get the cleanest frequency response for all listening positions.