Setup Speakers Along Long Wall?

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.

I typically prefer long wall speaker placement. Often long wall placement will improve bass response and provide a more spacious sound stage. There is one very important caveat; if the listeneing position now becomes close to the back wall, absorbent room treatment behind the listener is needed to avoid comb filtering effects.Thick foam covered with natural wool works very well for this.