Frequency Response

The positive for your situation is that the wavelength of the problem frequency is around 1m and to effectively treat with an absorber the rule of thumb is 1/4 wavelength thickness of material so 25cm or 10" which is pretty do-able. That might sound like a lot but it’s worse when your problem frequency is below 100Hz (>3’). You can also get away with thinner material if there’s an air gap between the absorber and the reflective surface behind it... the important thing is that the surface of the absorber is 1/4 wavelength away from the wall / ceiling.
You really want something dense like rockwool (I think it’s called owens corning in the US) and any covering needs to be porous.
Angling the treatment helps to broaden the bandwidth of frequencies that are treated (lowers the Q of the panel) but to trap 330-400Hz waves the 1/4 wavelength rule still stands (so you’d want the minimum distance from the surface of the absorber to the wall / ceiling to be 10").
Sound on Sound magazine used to run an article called ’Studio SOS’ which you can still find on-line, it’s quite a useful resource for creatively treating the acoustics of small spaces.
One last bit of advice - stop when it sounds right... you’ll never measure a fully flat response in a room with parallel walls, floor and ceiling.

The flooring between/behind your speakers may not fix your particular issue, but I encourage you to listen to music as you try it. :)

Awesome!  Thanks everyone. Wrik, I haven’t looked into the floor behind the speakers.  Will do!

@MC, thanks dude!  I was thinking theres a mathematical process snd you helped with that, Thanks.  Will research that more for sure.  I never thought about it until right now but yeah, with a wave you’re going to have different response in a rooms height not only width and depth.

I am using room correction software but it doesn’t matter for nulls.  People may think so but it really doesn’t, at least with the software I’m using.  Makes sense though because if its a wave cancelling itself out then turning up the volume does nothing for it,

@Oldhvymch, my response is a bit accentuated in the mids, just like you’d like it!  I prefer for the highs not to be rolled off so I’m enjoying the eq aspect of Somarworks.  I feel like its done pretty well.

Anyhow, Thanks for all the input.  Will def. look into treating the ceiling and with angled panels.  Will also look into that info you suggested MC, thanks!

 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.

It is a vertical problem. It is difficult increase volume in one area. Easier to attenuate volume.
If you put acoustic tile on the ceiling you will attenuate everything over 250 Hz and probably wind up with the same problem. It is called a "null."
If you have carpeting on the floor removing it probably won't help either.
The easiest way to fix it is Room Control. Check out my system page and DEQX.

How about the floor between/behind the speakers??

I think it could be a camber, caster issue to fix it. BUT the reason is probably the XO point.  I fixed quite a few by tipping them forward or backward. It is usually forward for a dip in the mids, and back if the tweeter is boiling my ears AND I want a little better Mid Bass response.  Start with 1/2" and then go all the way down to shems. The chair idea is a good one to tune it the rest of the way..

Just remember, a little mojo goes a long ways.. Flat has never made me happy. It's always been a very slight frown, looking after tuning it the way I like it,  I hear what I like.. Slight rise in the middle. When I measure (not to often) it's always the same.. a slight frown on the PEQ. 2 db rise is just about right..

Cup your ears and see if it's what you like (chair forward call or EQ it.)

Happy hunting..

Regards

@erik_squires , thought the same thing. I put a panel on the ceiling right above and forward of where my listening seat is, right where 350hz was loudest.  Didn’t do much.  Then I took 2 panels from the front wall and put them on the ground in front of the speakers... Nothin.

It may just be this crappy little room Im working with.

Surprisingly my bass response is good.  0db at 28hz and -3 at 24hz. 
My response is pretty crazy / peaks and dips.  Just shows how much work my room needs.  I am actually hearing strange reflections when the program is playing this clicking noise in order to locate distances.

Anyways, I may work on it some more today and see if I can figure it out.  Wish I had some 25 foot runs of speaker cable.  I’d just move my speakers around...

@newbee, definitely a null...  I just difn’t know or think that they happened above 100hz or so.  Never thought abkut hugher up frequencies cancelling themselves out.  Makes sense though.  I also didn’t realize how high of a pitch 300-500hz is.  
I tried some really big differences in speaker location and speaker height, even flipped them upside down and measured the freq. response.  Interestingly it didn’t change much.

I always knew the room makes a difference but now I am really beginning to see how much of a difference.  Its great to actual have a visual of my freq response now.
Anyhow, Im hoping to tame this and some other areas with more room treatments.  Maybe in the meantime I flip my listening position and see what happens 🤷🏼‍♂️

It might be floor/ceiling  bounce, but try throwing a blanket on the floor between yourself and the speakers as an experiment.

Also, I _highly_ encourage you to consider your ceiling when you can.

Best,

Erik

Sounds like a null related to chair location and room dimensions. Try moving your chair backward or forward 6 inches (more or less) and see if the frequency amplitude changes.