This is my setup...
http://i1345.photobucket.com/albums/p662/john_be_good/DSC04842_zpsc299eb4d.jpg

need to upgrade sub soon, due to space constraints do I go external amp & 1 sub or integrated amp & 2 subs, I do not have any space in the back or sides only the front.

in my rack I have pvr - dvd - mrx & beside rack; an xpa-5

2 subs, 1 per side, at the mid point on the long walls would get my vote.

Not possible on the sides, there is heating on one side and a door on the other. either I remove the xpa-5 from the mix and put a second sub there at the same time would need to change the underpowered amp or just upgrade the existing sub .... a friend suggested that I put the tower speakers on top of the 2 subs but don't think that is any good

perhaps if I made the vp180 shelf deeper and put the xpa-5 behind the vp180, it is triggered so no need to have it in sight, then I could 2 subs and have the external amp..

after taking a few measurements this isn't a good solution, at the end is it a bad thing to have just one sub?

I'd consider moving the towers away from the rear walls (sub behind amp?) and or a new stand for the VP180.
It looks like you've got an inch or 3 to work with (between the bottom of the screen and the 180).
My $0.00002

It's not bad at all to have one sub if it kicks butt.