[ErVikingo]

I'll try to simplify as it could take a lot of typing to explain it all.

Your room has modes at 31hz (dip/null at 8.7' approx.) 63hz (dip/null 12') 94hz (14'approx) 126hz (15 ft).. both lengthwise and widthwise.

When you say you are 12ft away, is that from the front wall or from the speaker's faces?

I would put my ears closer to 10' from the front wall to stay away from the nulls. By the way, sitting at the center (widthwise) puts you smack in the center of the null at 31hz and 94hz.

As to crossover setting on the sub, your Yamaha's go down to 55hz so I would set the sub at 65-75hz to get it to blend as the Yamaha's drop off.

These are just suggestions, I will try to give you ideas of positioning for the speakers