How many sq feet of diffusion do I need?

I think you mean they found some Nodes. I would do the same as you, do it in incremental steps. First of all, you need absorption on the side walls (the point of first reflection where sound from spkrs bounces off).
On the right side, definitely on the door and maybe a strip on the wall next to door. The left wall maybe 4x4' or 2x4 vertical. BTW, you actually need some reflection in all rooms.

Also, something on the REAR wall behind listening position will absorb standing waves. Room treatments such as these will make such an improvement; after that I would work on whether or not you need bass traps. AND I think you should shop around for the right kind of panel; foam would be much cheaper than the one you posted.

Gotta tell ya, you have a very nice system.

IMHO
Start with the bass issues first.
It's counterintuitive but fixing bass nodes significantly improves your rooms mid and high range performance.
Tube traps make a significant difference in the mids and high performance of your room. Once you eliminate the bass nodes, a lot of issues with highs get resolved.

I got a huge improvement in all aspects of my room when I installed a 16 x 48 ASC tube trap in each corner.

If you notice any type of boominess at all, I would start there first, then add diffusers later if you feel it's still warranted

Tommy,

Before you go spending money on diffusors, please try what I've done in my 10'x12.5' room.

Get six 2'x4'x4" absorption panels, place two behind you and place two on the side walls at the first and second reflection points.

Then get a larger rug for the floor between the speakers and you for the first reflection points on the floor.

Then assess whether or not you need some diffusion on the wall behind your speakers.

You might be as surprised as I was after I ended up with this arrangement. When you look at my system pictures, don't pay any attention to the two smaller 2'x3'x2" panels behind my speakers on the side walls. They don't do anything and are going to be removed.

One thing about diffusion. Ethan Winer states in his new book that if diffusion is needed and used on the walls, they usually should be vertical diffusors. Three-D diffusion sends sound waves to the ceiling and floor besides vertically, and sometimes adds to the reflection problem. Three-D diffusion is best, if it's needed, on the ceiling since it scatters everywhere.

Chuck

Right on, Krell_man. I'm using the same setup as you in my small room, except side walls are vertical.
And your point is correct, he needs absorption, not diffusion. Panels can be flat foam, wedged, but not 3D. I'm using 2" offset wedged.

examples

I think you either have to trust the folks who have heard and measured your room or not.

You are buying their recommended solution, so maybe ask them for the best way to work towards the complete solution step by step and look for improvements each step along the way. Or perhaps get a really good discount on teh whole deal and be done with it. A satisfaction guarantee would be nice especially if there is significant $$$s involved.

Not to complicate things, but wood diffusers like those referenced look like pretty expensive ways to accomplish the goal. Everyday household items can often be used to absorb and diffuse sound for much less, assuming aesthetic issues can be worked out. Diffusing and absorbing are two different ways to accomplish similar goals. Maybe try to add some things into your listening room to help break up whatever nodes or resonances might be occurring.

Or there are surely other commercially available treatments of various types (bass traps, absorption panels, diffusing panels, even both combined) for absorption or dispersion that are more economically designed and equally as effective. I have purchased such things off Amazon and found they need not cost much to be both effective and attractive.

Off the cuff, that room looks pretty spartan and more lively as a result with speakers not too far away from walls and likely in need of some combo of absorption up front, maybe also bass traps after that. Adding diffusing panels made of hard wood (rather than say absorbent foam or equivalent) that is also highly reflective of sound does not make sense to me from what I can tell.

Have you tried moving the speakers further out into the room for comparison? Did the consultants suggest or consider that as an option to selling treatments? More distance to walls might have a similar effect and help open up soundstage and imaging perhaps as well. Easy and no cost to try and see.