Setup Speakers Along Long Wall?

With the potential cost of good cables, I might first look into what’s going on w/ your turntable & why there’s a problem ? I use that cartridge but not familiar w/ the Technics. My Basis table / Vector arm is likely substantially better than it but it’s pretty close to to my speakers which I play loudly & have no issues. There are many fine, not crazy expensive turntables now w/ good suspensions that might work well. There are also many types of “after market” fixes to put your turntable on that might work too. I’ll guess there’s a thread on this site that addresses this. Just a thought. 

Long interconnects are always better but over 15 feet should be balanced.

NEVER put a listening position against a wall. Somewhere within the middle 3rd of the room is best. 

Check out the posts that mention AES48. My gear supports this so I use 15 foot Benchmark XLR’s that are low cost and sound excellent with this supporting gear. If If I was able to put a hole through a Livingroom wall I would use 50 foot XLR’s to get my rig completely out of the room.

Luxman gear, for example, does not support this standard and I would not use my Benchmark XLR’s with that, nor would I keep my amp(s) so far away from the preamp. First of all it would be very expensive to buy commercial grade XLR’s and I do not have confidence it would be good. Pro gear such as Weiss, Benchmark, CODA, and I think, Bryston support this.

One thing I noticed with LONG wall speaker placement in my Livingroom was that the speaker just 1 foot from the front wall was OK. I think this is because I have 7 feet to my left side wall and 5 feet to my right side wall. That right side wall is a partial wall that opens up to the family room. I also have 25+ feet of ceiling height in the Livingroom. All of the LONG wall space is what likely makes the sound work with a floor stander so close to the front wall.

In my last home, I used the long wall and ran the speaker cables under the floor. I’m a firm believer in not going long with interconnects, unless you are using XLR. And then only if there is no other choice. 
I did notice the lack of sidewall reflection and I grew to like it.

All the best.

JD

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.