Why does bass have more “punch” when I stand up vs. sitting?

My main speakers are flat to 20Hz. A standing wave is in the room with them- no bass at the listening chair, which I can't move (and if I did, the bass isn't in the middle, its to the side...). So I installed two subs (both are Swarm subs made by Audiokinesis, since they had to be as close to the wall as possible). One is to my left, the other more to the right and behind me. This asymmetry was enough to break up the standing wave; now the bass is fine anywhere in the room.

If you really want to fix this problem, this is really the best way to do it. Room treatment and room correction won't do it (but they help if the DBA is already in place).

 Suggestions as to how to have the bass response while sitting as I do when standing?  

@lourdes  Sure- you want to look into something called a Distributed Bass Array. This is a set of subwoofers that allow you to break up the standing wave that you are sitting in. The subs have to be places asymmetrically in the room. They must not have any output above 80Hz or so; else they will attract attention to themselves. They can be fed by a mono bass signal because an 80Hz waveform is 14 feet long; by the time your ears can even acknowledge the bass in most rooms, the bass has bounced off of every surface in the room, meaning its entirely reverberant.

The best subs for this purpose IMO are the Swarm subs made by Audiokinesis. They are designed to be as innocuous as possible- they are designed to sit inside the room boundary effect (IOW right against the wall) and so give bass flat to 20Hz. They have garnered three Golden Ear Awards to apparently others think they are pretty good too.