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

By standing, you are changing the position of your ears.  Bass modes—where waves are either summing together (increasing in amplitude) or cancelling each other (wave is out of phase)—occur in all directions, including up and down.  These interactions are so complex that there is no formulaic answer— you can only experiment with speaker placement, placement of your chair, try some corner bass traps, or utilize room correction/equalization, or install subwoofers.

Right. Bass modes aren't just left to right and front to back, they are also vertical. Another common thing bass is reinforced near walls and so almost always more full and round or even boomy, and gets tighter and more punchy as you move away, so standing up is also moving you away from the boomy floor bass towards the middle of the room tight punchy bass.

Sounds like a null in the room at the seated position; maybe from floor bounce. Experiment with rugs in between and something on the rear wall behind you.

Try raising your speakers. I had some Hales Revelation speakers that didn't come into "focus" until I put them on chairs. I have my Klipsch Forte's on 8 inch platforms.

 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.