Well, you are right in that a sub will add "something." The question is whether that will augment a lacking low end or simply add distortion by increasing the bass above intended amounts. I think there are two aspects to look at when making this choice.
1) Is the low end really deficient? That can objectively be measured. A calibration mic and software like REW can be utilized to run a frequency sweep and it will pretty much tell you where the low end is (or is not) dropping off. If it truly is, and speaker placement is not possible, then adding a sub is a viable solution.
2) Do you just want more low end despite the fact that it may already be accurate? I will speak from my personal experience. I have two independent setups in my listening room. A 2 channel for my music, and a 7.1 system for home theater. I use a sub in my home theater system and I actually have it calibrated to be above a completely linear frequency response. Why you may ask? Well, very simple. My wife and I watch a lot of action/sci-fi content and we actually enjoy the rumble of those big explosions, light sabers clashing,... etc. I know that I am introducing distortion into how the original soundtrack was mastered and that's ok. It's a calculated and wanted distortion. But in my serious music listening which is done at more reasonable volume levels,... absolutely not. I want that pure as possible to the recording.
Ok, so those are the two factors and I would go through the "hassle" of finding out which group you are in.
I would recommend this calibration mic if you don't already have one:
https://www.minidsp.com/products/acoust ... B4QAvD_BwE
If you do decide to go the sub way there are countless choices. I think Margin Logan really makes some wonderful subs so I would look at those if you have the opportunity. In the US, I think Hsu Research gives amazing bang for your buck when it comes to subs and I use one of their VTF-15H's in my home theater setup. If looking for a very detailed and subtle enhancement, I would maybe even look at the Magnepan DWM bass panel.
What I don't recommend is putting a cheap sub in the system as at the end of the day, the sub becomes your speaker and you don't want a quality mismatch between the sub and the rest of the speakers. Rotten grapes will spoil the entire fruit salad.
