Bass only when I stand up…

Bass is more non directional. When you stand up the mids and highs are going to be potentially much less. Think that could have anything to do with less perceived bass from standing? 

The quickest way, not the only way. Some of the suggestions will help. First pull your speakers out from the wall. At least 500mm, next add subwoofers. If it's not in the budget pick up 2 used woofers for the time being. As you know bass is non directional, adding more fills the nodes in the room and helps equalize the bass in the entire room. It isn't about getting more bass,  you are simply filling the nodes in the room. You may have to turn them very low to help from disrupting your main speakers, but it will work. Good luck, Tim

Sorry 500mm more. 3 1/2 to 4ft total distance from the wall

Just get a booster chair 

Where the heck did my last response update go?

Well, good news.

I took the advice here about moving the speakers closer together. I moved them about a foot each side towards the centre and about another 6” forward.

Problem solved.

Now there’s no difference standing or sitting. Both have the same excellent bass. Powerful, deep and clear. Wife approves! She’s a long term Hi-Fi buddy.

I have them towed in towards the seating position and it’s a nice clear focus. I’ll bring another eames style chair in this room shortly. Or an exercise bike!

First, thanks to those here who suggested this solution. It worked. I’m grateful.
To Erik and others here who provided detailed analysis and thoughtful suggestions I appreciate your efforts. Maybe these are constructive suggestions for the future.
​​​@danager try closing the gap between your speakers as a starting point as much as you can. Let us know what happens or what obstacles you have.

John