Subwoofer 3D Illusion

I have a set of floorstanding speakers, Silverlines, that do a good job at reproducing bass; however, I wondered if a subwoofer would add a more phyiscal presence to bass drums. I bought an economy sub off Audiogon to see if it would do the trick.

I listen in a somewhat of a nearfield arrangment. The mains and my listening chair form an equalateral triangle. I recieved the sub and hooked it up to see if it worked. In the test to see if the sub was mechanically working, I just placed the sub behind my listening chair. Afterwards, I was going to move the sub to a more typical location in the setup, but a strange thing happened.

I have the sub crossing over at the lowest point possible, so it doesn't do much of anything except when percussion kicks in or maybe a piano hits the low registers.

The effect is that mids and highs still come from the mains as before, so does bass, around 40hz, but when something in the lower registers hits, below 40hz, it seems to come from all around, giving a pseudo-larger soundstage. The song Changes, on YES 90125, is a good example. Without the sub, the drums at the start sound great. With the sub it seems to have a more theatrical, dramatic effect.

It is a weird kind of trick. The mains are in front and the sub is right in back of me. The fact that the sub is right next to my listening chair makes me feel the bottom in my gut more than before. I am not sure if I like it or not, but it is a trip for now.

Has anyone else tried this?
At some low frequency, determined by the spacing between them, the sub and the mains are driving the whole room in "push-pull" manner.
My sub is behind me also, it is the recomended placement from HSU Research, my towers go to 18HZ so I dont use it much for music unless classical is in the CDP but for movies it gives me a more 3d image, now I can great bass from front..and rear
They are Vintage Teledyne Acoustic Research AR-9, I had them completely went over 3 yrs ago, they have a 4-way design 2 12 inch woofers, 8 inch lower mid, an upper mid and a tweeter....they are old, but sound pretty darn good to me, although I am looking to replace them soon with VMPS monitor2 speakers. When new they went for $1600.00 wich one can argue that might translate into around a $4000
speaker at todays cost, I am new expert though, but that would be my guess...they bluebook at around $600 still.
I had to follow up on this post. I decided to tweak the subwoofer by removing the rubber feet and adding spikes, and opening the sub, replacing the foam with audiophile egg carton stuff I bought off the internet. Last, I placed a marble slab from the garden on top of it. I still place it in back of my chair and it is sounding better than ever; however, the 3D effect seems to have disappeared to some degree. Now, the bass seems to be coming straight from the mains which is kind of funny considering the sub is actually behind me, out of sight.

I am playing the new release from Bad Plus and it is really, really freaking me out.

Now, I am wondering how all this would sound if I have a true, audiophile grade subwoofer.