Open Floor Plan Listening Room

The bigger the room the better for deep bass. Irregular shapes also help, as was already pointed out. From the description, it sounds as if you have a null at the listening position. If so, the first thing to do is to measure the response in various locations to make sure that this is the case, with, e.g., REW. After all,  the louder bass you hear elsewhere in the room could in fact be bass peaks. But if there is indeed a null at the listening position,  it cannot effectively be equalized/boosted. The solution would be multiple subs like the Audiokinesis Swarm system.

There are two ways to wire subs. The first is from the pre out, and if that is XLR and the sub(s) do not have that, you will need an XLR to rca cable. If the sub(s) have XLR, use those inputs. The alternative (not available on all subs) is to connect them at speaker level. REL are strong advocates of connecting this way. Nearly all subs have their own inbuilt low pass crossovers. Some also have a high pass facility to relieve the main speakers of the duty to reproduce the demanding low frequencies. This makes for a slightly cleaner sound from the main speakers, but makes integration harder. If the main speakers are full range I would not bother. It is usually best to use multiple subs in mono rather than stereo, as you can more easily equalize them succesfully.
Sometimes you have to combine speaker level and line level connection. If you were to use a DSpeaker Antimode 8033 room eq for the subs (as I do and strongly recommend) it only has a line level input. If you do not have a pre out, you will have to get the signal from the speaker level output, and use an attenuating cable to reduce the level to line level. Fortunately none of this is complex, and no fancy cabling is required.

I have two recommendations to make. The first is to go for at least two smaller subwoofers instead of one big sub. In most situations that produces a far more even response. See here for some reading: http://www.acousticfrontiers.com/20101029using-multiple-subwoofers-to-improve-bass-the-welti-devanti...
The second suggestion is to invest in a DSpeaker Antimode 8033 room equalization unit as well. This tightens the response significantly in both the frequency and time domain. With two subs this is even more effective because the equalization will be effective over a larger listening area. Subs have a bad reputation in audiophile circles, but this is largely due to the problem of the room modes that they excite. These not only produce peaks and dips, but they also linger on. People speak of slow subs, but it is not the subs that are slow, but the lingering room modes.

But what if it does? The description seems to be exactly a null, which could indeed be the case if the listening position is halfway the length of the room. But it could be that the louder bass elsewhere is actually a peak. So as I said at the beginning, measurement should come first. The only investment is that of a $75 calibrated measurement microphone.

This is an important reminder that no domestic space will ever give you the acoustics to mimic a concert hall. It will always be too small, limiting the dynamic range that can be reproduced, and creating havoc at lower frequencies. A good postage stamp image of reality is the best we can hope for, and all the more so with large scale dynamic music like a symphony orchestra. Here, the real thing will always remain better.

I would suggest an Antimode 8033 room eq unit to smoothen your subs even more. They are cheap, easy to use, and highly effective.