Bass only when I stand up…

Weird experience.

Wired up my system and sit down in my office chair. The bass disappeared. Stand up, and alakazam, the bass is back full and wonderful. It’s like swimming in the sea, there are horizontal layers of different temperatures. Quite distinct.

Ok, system info:

Speakers - PMC MB2SE

Power amp - Marantz 8807

Preamp - Marantz 8005

Sources - Pioneer LX800 or N70ae

Room - carpeted, 4.27x5.5x2.75m, acoustic insulated plasterboard and timber frame

Shape? Left side where door is, has duct risers that create 800mm boxes either side of the door. I’ve tried attaching a photo but didn’t work. Try here: link

Speakers placed 600mm out from the 4.27 wall, with the opposite wall being a library and office desk.

Tried? Raising front and rear speaker stands 1” to point speakers up or down slightly with no effect.

I welcome ideas 💡 yet realize there are constraints here too.