Maybe I don't understand your problem. Standing waves ( as I recollect ) is the sound striking the rear wall and washing back into the main sound field from the speakers, causing errors and cancellations.
The main culprit of this is the lowest bass frequencies, which require more room length to propagate before meeting a barrier.
I would guess that the deepest bass possible from your ATC active 50's should require no more than 32 feet from the speaker face to the barrier (rear wall). This is a generous assumption, and considering that you play very deep musical content, such as pipe organ.
You probably have a different problem. Please fill in a little more detail about what you are hearing and what it is you don't like.
The main culprit of this is the lowest bass frequencies, which require more room length to propagate before meeting a barrier.
I would guess that the deepest bass possible from your ATC active 50's should require no more than 32 feet from the speaker face to the barrier (rear wall). This is a generous assumption, and considering that you play very deep musical content, such as pipe organ.
You probably have a different problem. Please fill in a little more detail about what you are hearing and what it is you don't like.