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

Tonywinga.  Thanks. Did you change the angle (i.e. toe in) of your speakers as well?

Yes, but only by 1/8”. 

As many have already advised, you are most likely sitting in na bass null. I had the opposite issue where I had a bass node in my seating position and it would disappear when I stood up. I used an online room mode calculator and voila, I had a vertical bass node. Suggest you try the same. Wont cost you anything. Just get your room dimensions and put them in to a room mode calculator and it will show you what's happening. In my experience, you cannot get rid of the null unless you change your seating position. Or like someone else also suggested, get 2-3 Subs around your room, which will help improve the situation. 

standing also reduces the relative level of mid and high frequencies making the bass appear louder in addition to the room modes.
best results will happen if you can move your speakers well into the room, 4 feet to the wall if possible.  also if you can move your seated position closer but not too close to the wall behind you.  
if your positions are locked then a sub or two is a good option.  

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).