Anyone have experience with acousticfields.com or with using true pressure traps?

Not yet, but I am super interested.

Do you have a way to excite the room and measure the pressure (frequencies) in the corners?

You probably need ^that^ to get the traps built to address the frequencies you want to suck up.

Get a mic and REW and do some measurements in the space.

Are you thinking of the mass loaded membrane traps?
Those are somewhat modellable, but I think you need to have the target freq, and then tune it by adding or taking away a bit of mass at the end… much like a passive radiator.

There are also the round bass traps, which could be added later, as well as active bass “traps”. But starting with the in-wall resonant ones makes a lot of sense.

Once you know the low frequency decay, then just treat the higher freqs so that their delay is similar. (I.e. don’t go overboard with super short ring down times for high frequencies, if the low notes are taking forever to decay.)

The resonant freq on the various places will likely be different along the walls… !

Lowest in corners, then long walls, then short walls.

Usually the most optimal solutions are mostly passive control, with some active control layered on top.

  It does remind me of a product though that didn't go far for room correction. Must have been a financial thing because it made sense to me. I remember some company that used correction in the form of an anti-bass wave being used to cancel out the nodes in the room. Basically a sample was taken during every moment of the music and a microphone and amplifier/speaker combination produced the anti-wave. Seems this system was placed in the corner of a room. Didn't hear much more about it after its inception. 

And using that active control, following the passive bass traps.

... It appears that speaker/listening room placement is the only way to minimize the effect of nulls?

Correct, a null and a peak are destructive interference and a constructive interference respectively.

However the room dimensions will define the peaks that exist in the corners and parallel walls.

This is why I mentioned a microphone in post #2 and #4. One can calculate what they should be, and where. One can also measure what the peaks are at a variety of “where” positions. Knowing that, then you have a map of where the modes are high that need to be absorbed, and what freqs they are at.

We do not get pressure nulls in the corners as easily as we get them in the listening position(s). (We get velocity nulls in the corners and walls,) And your treatments will be located at the walls, so you have the problem constrained there at the wall, and only need to measure there. (But measurements at the listening position should likely also be done.)

As the long modes will be longer, they will be at a lower frequency… and at a lower freq than say the short wall distance modes. And if the traps are low-Q, then they will largely suck out what they should. if they can be made with a lower Q and tuned in-situ then it get easier to be closer to perfection, but the low-Q obviates the need for being “overly perfect”.

Treatments fix the peaks. And yes placement the nulls.
So focus on the peaks for the wall treatment.