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.

Each frequency put out by your speaker has a different wavelength determined by the speed of sound in air. A wave reflecting back and forth between two parallel walls will reinforce itself if the nodes of the wave coincide with the room boundaries. Thus your room selects a set of frequencies coming from your speaker to reinforce. If you sit at the PEAK of a reinforced standing wave (also called a mode) that frequency will appear louder. If you sit at a null of the standing wave, the frequency will appear to be sucked out.

So EVERYONE listening inside rooms with opposing parallel walls has a standing wave problem

This is an interesting topic. I have a Mac 2000 amp, Mac C100 pre, ECM-1 CD player with Nautilus 803's. Pretty good stuff, but those standing waves. Talk about nulls and modes. My room is 11 X 12 and is kind of small, but comfy. I treated the room with Echo Buster bass traps, diffuser and absortive panels. These helped a bunch, but still problems with placing my one chair in the room and speakers to avoid areas of total lack of some frequencies and over emphasis of others. Any ideas where to place my chair vs. these speakers?

Amwarwick: Check a thread entitled "Near field listening and speaker placement" In the various posts of this thread are references to several useful websites which address exactly the question you have asked

Will check this and thanks Judit.

One foot from the wall??? Say it isn't so. I assume since your room is 15' wide you mean the ATCs are 1 foot from the front wall. If you can I'd say get them at least 2 or 3 feet from the front wall and toe the speakers in a bit. A pair of Mana stands may also help. Also, make sure your listening chair is at least a couple feet from the back wall, as that will also reinforce bass at the listening position.

If all else fails, TacT room correction will certainly take care of the problem, and many others as well. Best of luck.

Tim