Standing waves

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.

Actually, this is a simple case of improper speaker/seating LOCATION LOCATION LOCATON!....plus a lack of some expert set-up assitance that is needed.
I've done over 1000 systems in my time as an audiophile/custom theater designer, and the stone cold reality of the matter is that 99.99% of all people really don't know how to properly set up ANY speakers/system in a given domestic room(let alone an expensive one),thus the never get any serious sound quality..but rather mediocre to lack luster sound quality.
With even the best gear you can throw money at, all Class A rated gear etc, set-up and placement in a room is definitely were everyone falls short. The proper room set-up and acoustics are at leat 60% ofthe sound you hear, and the downfall of most audio enthusiests...just takes time, knowledge, exeperience, and abit of expertise to really pull of world class sound...something most people just haven't acquired. It's amazing actually what moving speakers and such around does for the sound in any given room! You can't, unfortunately, stick gear in a room(at any price), and expect it to sound good. It just doesn't work out usually.
Anyway, you definitely need to move things from where they are, just by the sounds of it. If you must keep certain things in certain locations, you must work around it, and try to get the best location for either the speakers or listening seats, preferably both.
Anyway, good luck. With speakers that expensive, you should DEFINITELY be enlisting the help of a professional who's competent at setting up speakers/rooms at least....this is the difference between making or breaking your investment! Sounds like you've already made the equipment part. Now you just need the other part(the one most poeople don't consider unfortunately).
Hey, just outta curriosity, where did you get your ATC's???

Oh, sorry, you definitely have standing wave challenges in EVERY ROOM actually!..not just yours. You or someone needs to know what's going on in your particular room, and place things so that you work WITH the standng waves, so you get flat natural frequency response, instead of either boomy, peaky, or uneven frequency response.
By the way, love your speaker sellection, you have great potential there.
Good Luck

Make them sit down.

Celtic66 it's nice to see someone is trying to pick up the slack now that Cornfedboy is gone. But don't worry in time it will all pass.