Total bass suck out at 40hz

Sub crawl. Put one sub exactly at your listening position. Play the 40 hz tone on continuous repeat. Get down on your hands and knees, and carefully cover the whole of the listening area--some kind of grid pattern is good so you know where you’ve been. Mark any spots where you can actually hear the tone. Hopefully there will at least two. Move the two subs to the marked positions, and return to your listening position to check result.

I wonder how many have actually done any of the stuff they recommend. I did the sub crawl. Subs wound up right back where they were, along the walls, asymmetrically different distances from the corners. Tim did the crawl, his are in the corners, same thing. Its no big mystery. Bass is ALWAYS strongest in these locations! 

Want good bass? Put a sub near each corner. 

We now return you to your regularly scheduled missing the point comments. 

I do not have enough cable to place subs at my listening chair. My room is very long at 38' only 14' wide. I bought a pair DL Labs sub cables here on the Gon at 12' each, also hampered by outlet locations. 

Mizike, leave everything as it is for now. Have you measured at different points in the room? It sounds like you are measuring in a null. Putting subs next to your sofa might be great for Movies but really bad for Hi Fi.
Subs in corners is alway a good idea as long as you can match them in time and phase with the satellites which without digital bass management you can not. So, with a point source system the best way to set up two subs is against the wall between the satellites on a radius from your listening position. This means that all speaker are the exact same distance from the listening position. Assuming none of your equipment inverts phase you should be pretty close. You will still have node issues in the room. The easiest way to deal with this is by moving your listening position forwards or backwards to a position were the bass sounds best.
without room control that will be the best you can do unless you are ready to add two more subs. I would keep them on the same radius but against the side walls. Hope this helps  🥴

Mizike, it is not uncommon to have room modes and boundary effects combine resulting in nulls of 30 dB or so.  That pretty much constitutes a total suck out, as you put it.   I assumed you have 8 ft ceilings and put your room dimensions into the amroc room calculator.  You have 4 axial and tangential room modes between 40 and 50 Hz.  Those will be broad bands and will merge into on another, resulting in a deeper null.  It's possible you have boundary cancelations due to speaker and listening position adding to this problem.  If you google Speaker Boundary Interference Response, you will find SBIR calculators that will help you figure out if speaker position is adding to the problem.  You have gotten a lot of good advice above, but I suggest you approach this systematically.   Also, you must be aware that you can't just improve the 40 Hz null without impacting other frequencies.   What you want is not the flattest 40 Hz response.  What you want is the flattest response from 20 Hz up to about 150- 300 Hz.  If you decrease your 40 Hz null by 20 db but create a null of equal magnitude at a different frequency, what have you accomplished?  So as you work on improving the 40Hz null, pay close attention to what else is going on at other frequencies.

First, optimize the positioning of your Yamaha mains with the subs turned off. Without explaining why, I would try you speakers placed with the center of the drivers 34" from the side walls.   Because your room is atypically long, an optimal distance from the front wall and an optimal seating distance from the rear wall won't be practical.  You are going to have to play around with this.

Second, optimize your seating position.   I would advise not placing your seating position in the center of the room.  16 ft or closer to the front wall, or else 23 ft or further from the front wall will probably work best for you.  Once you optimize the seating position, move the speakers again a moderate distance forward and back, and make sure they are still placed optimally.

Third, optimize sub placement.  Asymmetric placement is best.  If you set your crossover at 80Hz, you won't need to worry about loss of image or time related aberrations.   If it were me, I'd try at least one of the subs elevated off the floor, perhaps as much as 4 ft. Try one in a front corner, then try it centered between the speakers.   Try a sub along a side wall out as far from the front wall as you can get. 

I encourage everyone who gets into this to think about investing the time required to learn to use and interpret REW.  The ability to rapidly run full spectrum sweeps makes room optimization and speaker placement much easier once you get past the frustration of getting started.