Placement of Single Subwoofer Affecting Sound of Main Speakers

+1 jl35's crawl test suggestion.

Most any room has an optimum location for a single subwoofer that will result in efficient performance at the listening position. 

You'll need a pair of long interconnects and an extension power cord. Monoprice or Blue Jeans cable offer custom length interconnects at reasonable prices.

Playing a test CD or file with a low frequency cadence will aid in hearing the loudest area or mode in your room.

You don't necessarily need to crawl around your room but it can provide a more dramatic sound.  

Positioning the subwoofer at or near this area should provide a satisfactory location to begin the other adjustments. Good luck with the aesthetics.    

Ideally you want one sub front and center. Front and slightly off center is next best. Avoid the corners. Rear center is another decent option especially if you sit closer to the sub than the front mains.   You can get away with a lot with sub placement though when needed especially if sub is set to not do much above 60-80 hz or so. 

Hey, and yes, but I didn't know it could happen with a direct radiating subwoofer. :)

The effect you may be having is that the sub is acting like a tuned bass trap, OR you are finding it's reflecting a lot from the speakers.  The latter can be tested with a blanket.  If the blanket covering the sub fixes your problem it's purely an acoustic reflection issue.

If it sounds about the same it's probably because the sub is sucking an octave out of the room.

Thanks for all responses. I find the below from Mapman particularly useful.

Ideally you want one sub front and center. Front and slightly off center is next best. Avoid the corners. Rear center is another decent option especially if you sit closer to the sub than the front mains. You can get away with a lot with sub placement though when needed especially if sub is set to not do much above 60-80 hz or so.

The corner placement with a single sub mirrors my experience. The current placement at the rear of speaker slightly away from the corner yields much better result. However, the effect or impact of a working sub is minimal since I have to set the crossover quite low at around 50 Hz. I tried setting it up to 60-80Hz and the sub is doing more harm than good with the bass sticking out like a sore thumb. Turning down the volume helps but there’s very little to no difference with the sub switched on or off.

I will likely try the 3rd option by pushing the sub further out to the centre of the room / speakers but it cannot be directly at the centre. When I tried this placement about a week ago, I recall the sound to be good but again the impact of the sub was minimal. Pushing the crossover and volume up too much and the bass sticks out too much. I will attempt this placement again once I receive the Nobsound springs.

I understand 2 subs(or more) are better but it is out of question. It is either 1 or nothing at all.

Should ask what sub are you using and what speakers? Also consider using white noise and a sound meter ( app on smartphone) to get an idea of what is going on in regards to sub filling in lowest octave at your listening location smoothly. Some speaker sub combos may not be matched well to accomplish that.