I don't agree with Jims "golden ratio" speaker positioning formula but do believe he is right on the money with this idea- depending on type and size of rack.
The best sound stage I have heard was at a dealer that had nothing between the speakers but a wide set of thick theater curtains.
A rack or screen in the center unpredictably and non-linearly mixes and reflects the omni directional sound waves and will not be optimum.
A single tier open rack is not as bad as a triple tier closed off rack that can also adversely reflect and fortify bass frequencies.
I am currently using a three wide IKEA closed rack and realize the sound is not the best it can be.