Room treatments - GIK Vs Acoustimac

Milpai,

The less you use to 'correct' the room reflections the better.

"...I want to get my system to a point where even when
I move my head from the center position, the vocals
should still appear to come from the middle.
Is this possible?

How should I decide between diffusion and absorption?..."

NEITHER !

Suggest you remove all your cross-overs circuits
from your speakers, change over to a non-crossover
circuit design.

The problem with room reflections is that the speakers are connected to the audio source with old-crossover circuits. The audio signal is changed at ever frequency, being phase distorted.
This 'phase-change' is noticed as a change in timbre of the vocals and in the unique sounds of instruments.

The acoustical energy should not need be focused to a sweet spot. A sweet spot is actually an indication of something not being done right.

And yes with the correct inter-face circuits you can
get away from the 'ideal spot' and end up with a much broader listening area; with vocals in their original 'positions' and instruments heard as never before!

The room's size becomes less of a problem due to correct
'phasing' of the audio signal(s).

Well let us start here...

I would NOT recommend ordering from Acoustimac.  They shipped me a partial order.  I have contacted them twice to ask them to ship the remainder of the order and have not received a response.  Disappointing customer experience.

I've noticed ASC products are not mentioned on Audiogon. Why is that---the higher prices? Their Tube Traps are "real" bass traps, unlike the flat panels from others. :-)

Don’t overlook the tiny little bowl acoustic resonators, there’s the original Tchang bowls, the Synergistic Research jobs, Ziplex and others. One interesting thing about these teeny tiny bowls (1” diameter) is how much they improve bass response. Also, special shout out for Golden Sound Acoustic Discs for room corners.