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

Once upon a time, I had my Tonian Labs speakers and listened to them on my overstuffed, microfiber sofa with built in chaise. I was never satisfied with the base. I sold the speakers and then got new chairs before I shipped the speakers.

With the new chairs (Knoll knockoffs) the base improved to the point where I wanted to cancel the sale but the buyer was hot to get them. Turns out the sofa was killing the base performance by absorbing too much of it.

This leads me to ask you what are you sitting on? It could be your furniture and/or it could be your room.

All the best,
Nonoise

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.