I just difn’t know or think that they happened above 100hz or so. Never thought abkut hugher up frequencies cancelling themselves out.Happens at every frequency. Every wave hits a flat surface reflects and all the reflections interact with direct waves to cancel or reinforce. Happens all over the place. Its just that usually there's so many things in a room the reflections are going every which way and its hard to notice.
Way back when I was doing my room nobody in Seattle knew the first thing about any of this and I had to figure it all out myself. Fortunately this is one of many, MANY essential audio items explained in Robert Harley's The Compete Guide to High End Audio.
Everyone today wants to go the easy techno route with microphones and stuff but you can do it all just the same with paper and pencil just fine. Diagram your room then start looking for modes using either math like I did or you can cheat with one of these http://www.1728.org/freqwavf.htm
Where if you plug in 350Hz you will find the wavelength is 38.6" which perfectly explains why you hear it sitting but then stand and its gone. How much ya wanna bet the diff between ears sitting and standing is about a half a wave, or 2 feet? Its all physics, see?
In my room early on it was an empty rectangle, not a thing in it, and I could play test tones and some of the higher frequencies would be completely different one ear vs another. Because as everyone knows I have a big head, not quite 38.6" though, it was probably more like 13kHz.
One way you can control that 350Hz peak is to break it up, which by now you can probably figure out is going to involve some panels that are around 3 feet square, which angled slightly will send those frequencies off in different directions breaking up the mode pattern you have now. Don't quote me this is off the top of my enormous head which has had so much stuff stuffed into it for so long now half has sloshed out but even so as you can see a few things still sloshing around in there. Get the book, or search around, you know its out there. It is after all basic wave physics.