Placement of Single Subwoofer Affecting Sound of Main Speakers

Hello,

Besides all of the good things to try above I would also check the Phase of the sub. If it’s out of phase it can really mess with the sound if it’s in a bad location. Sometimes there is a switch on the sub itself. If you don’t mind spending $150+ IsoAcoustic makes a sub platform that come with spikes. I think it’s called Alperta Sub. It works great as long as you subs don’t have a driver on the bottom of the sub like a REL T9. I think putting the sub in the listening position and crawling around is your best option. People sometimes turn the sub too loud. Turn up the crossover to 60 hz and turn down the volume until it disappears from its location. 
If you are in the Chicagoland area this store is an IsoAcoustics dealer and they can help with the sub placement. Maybe a wireless kit is the trick. 
holmaudio.com

They let you try before you buy something in your home. Very easy to deal with. I hope you can get the sub to work. It really made my system great. 

Hello Ryan!  The sub will absorb energy since its cone is moved by the sound in the room (especially bass) and since its cone moves, it generates sound itself even though its just sitting there not connected to anything. That's the problem with listening rooms with multiple speakers at a dealer's showroom. You could just put a pillow in front of it, connect the two terminals together, move it out of the room, or throw a rug over it. If you decide t use it, be sure it's in phase with the main speakers. Listening and then switching the leads listening some more will tell you what's right. Good Luck.

Have the sub to the left of your left floor standing speaker or to the right of your right floor standing speaker. The sub parallel in line with either. And at 0 phase. With the best crossover chosen(60 Hz? 40Hz? 80Hz?) for your room....I dial between 50Hz and 70Hz. And the volume low but just enough. 

That will give you the best chance with 1 sub as that is what I have currently. 

An update. To cut a long story short, the sub is staying. I’ll keep this short and avoid being long-winded. Most experienced folks would have known about this. After moving the sub around, I figured out that the placement of the sub is everything. The difference it makes is just massive.

As expected, the current best placement of the sub is between the speakers. It’s not at the centre but closer to the left speaker. I also found that the front of the sub needs to be on the same plane as the main speakers after moving it front and back. The sub now has about 2 feet from the rear wall (previously 5 inches).

With the new placement of the sub, the volume and crossover settings are completely changed. After trying several settings with different music, I’ve settled with these. The crossover frequency can now be turned up higher without the bass sounding forced or unnatural. Please feel free to comment if the settings need some adjustment. The volume and crossover settings are most important, phase setting is least important as I’ve always left it at 0, presuming the sub is in phase.

The volume is at 75%, rather high.

Crossover frequency is currently at 62 Hz.

Phase at 0.

I’ve also replaced these lowly interconnects

with some decent ones as below. Van Damme cables.

I’m not sure if it has made a difference but the new one is certainly more sturdy than the thin flimsy wires I was using before this.

The Nobsound isolation feet will be the icing on the cake.