You don't need to wait for your Acoustic Zen cable to come back to perform this testing. You can use Radio Shack speaker cable ($5-$10), or even some plain old lamp cord for the sake of trouble-shooting.
Why would you not install the speaker spikes??? Correct that wrong first, that should make as much of a difference as isolating your turntable. If the issue is feedback going through your system, fix the spike issue first, then if there is still excessive woofer excursion, look into a turntable wall mount.
If you are still having issues, then follow the switching cables path suggested above to trace down the guilty component.
Why would you not install the speaker spikes??? Correct that wrong first, that should make as much of a difference as isolating your turntable. If the issue is feedback going through your system, fix the spike issue first, then if there is still excessive woofer excursion, look into a turntable wall mount.
If you are still having issues, then follow the switching cables path suggested above to trace down the guilty component.