Setup Speakers Along Long Wall?

15 foot speaker cables aren't a problem. I use them with no ill effects.

Once you've solved the pesky low-frequency rumble, you won't have to worry about how to position your equipment. This is a great way to completely solve the problem. You can use the Energy-Pump power cable with your power amplifier. It removes low frequency rumble on many systems.

In a large proportion of audio systems, the low frequency rumble is produced by the uncontrolled mechanical inertial vibration of the speakers because the power amplifier does not have sufficient control. An Energy-Pump power cord can solve this problem. Why not try it out? If it doesn't remove your troubles, you won't have any cost.

The problem comes from the energy put into the floor (2nd story) by the REL S/812 subwoofers.  I solves it by placing the turntable in the corner where floor vibration was at a minimum.

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.

From my experience I agree with Baylinor. When setting up my room (using the long wall for placement) I was not able to overcome the reflections. Heavy speaker toe in made things somewhat listenable at low volumes but everything fell apart afterwards. I have no direct experience with using a turntable in this regard.

Rotating to the short wall, applying heavy window treatments and moving the speakers further into the room made all the difference. Ultimately I added diffuser panels, one on each side wall and one on the ceiling to tighten everything up. Since making these changes the only improvements have been with my adjusting seating position forward/backward but I only do this when auditioning new equipment.