Setup Speakers Along Long Wall?

You definitely want the TT to Phonostage short. Secondly it is good to keep the speaker cables short. A very common solution is to place the amps close to the speakers and move the rest of the equipment to the side to keep it from interfering with the speakers imaging. 15’ is not to far for single ended interconnects. But if it allows you to get a better preamp…we’ll, better is always better.

What does your venue look like? Speakers? If you put photos of what you have so far under your UserID it would be helpful in allowing us to envision your space.

Nice system. A great looking room.

Looking at your system my guess is the subwoofers are overloading the room with bass. I assume with the subs turned off you do not have the problem with the turntable? I would start here. 
 

I am fortunate to have a large room. As @curiousjim points out, having wide spaces on both sides of the speakers can be good. You’ll probably need to heavily dampen  the front and  rear walls and the ceiling. It is worth a try… but I would first figure out how to stop the feedback. 
 

I’ll give it some more thought. I am sure with the additional info you supplied, there will be some folks that will have helpful suggestions.

Typically the problem comes from footfalls on a hung floor. The solution for many decades has been mounting the turn table on a wall mount. There are many such solutions on the market. This will allow you to place your turntable wherever you want.