Thanks for the video and drawing, they are very helpful.
A few important things to consider:
You are setup on the long wall which should give you very good bass.
It looks like speakers are too close to the front wall. Try moving them out into the room so the back of the speakers are at least 2 feet from the front wall.
Search this forum for ‘Sumiko Masters’.
You will find detailed directions on how to setup your speakers for best performance in your room. Follow the detailed instructions and you will be rewarded with better sound.
Do this with the sub disconnected and preferably out of the room.
A few other things, less important but worth considering.
Get rid of the coffee table or cover it with a soft blanket when you are listening. This will improve tonal balance and imaging.
You have beautiful wood floors. Consider adding a carpet or rug between your listening position and the speakers.
Consider installing acoustic panels on the sides of the TV, behind the speakers.
Sometimes an excess of treble can be perceived as a lack of bass and all the above suggestions are intended to reduce treble reflections.
Check the polarity of all the speaker connections. If the bass drops when the balance control is centered you have got one set of speaker cables swapped. It happens.
Also it looks like you are using bi-wire speaker cables, but have the jumpers installed on the speakers.
Once you have the speakers dialed in, then add in the sub and follow the REL instructions for sub integration.
Good luck!