Yes. It can be done. The ARC software will autodetect if you connect the LCR via preout. With the exception of the center channel (a single channel), you can reassign the vacant front left and right channels to other speakers like your Atmos speakers to take advantage of the higher power output (140W) as opposed to 60W for the original height channels.
Where did you get that info? MRX1140 is considered a Flagship AVR by Anthem and preouts for all channels to connect to a separate power amps are available for all models.
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I got the information from a group on Facebook called: ANTHEM AUDIO LOVERS.
No, they are always active but as they will not be driving a speaker the power amp will not draw much current. On older models x00 and X10 you could physically Disconnect the power amp section internally.
Yes, I saw your post in the group. I’ve also replied in that post. Thanks to the dealer Corey Hall who made clear on this.
This short tutorial is for those who wish to connect the Anthem MRX or AVM via WiFi.
Congrats on your new toy. Im sure it will bring u many more smiles hehe.
Here’s a review from Stereonet UK on the Anthem MRX 1140.
Unboxing the entry model, MRX 540
Yo bro, thanks for the support and your time! Best of luck in your calibration and hopefully it will be as smooth sailing as the wind blows from the fans 
Thanks so much for the fans. Also thanks for letting me experience a real home theater- simply amazing your setup!
A very meaningful prologue for all Anthem ARC users out there…worth reading.
Here’s the extract:
"Anthem Room Correction offers a great deal of power and flexibility to adjust the target curves for your system. For the audio enthusiast, this is just plain fun. We get it. If you’re tempted to run ARC for the first time and start tweaking your system right away by adjusting the settings, consider the following advice. A great deal of research has gone into the algorithms that drive ARC and calculate parameters for the target response curves and bass management settings.
We strongly recommend letting ARC adjust all the settings and spending some time listening to your system before diving into the deep end with manual adjustments. When you’re ready to proceed, please remember that most Anthem receivers, processors, and preamplifiers allow you to create multiple profiles. If you are unfamiliar with the more granular controls within ARC (which can sometimes be confusing at first to novices and experts alike), let ARC set all the controls in the first profile and create a second profile where you manually tweak the settings. Upload both to your audio equipment and listen! Switching ARC profiles in your device’s menu is easy. You may be surprised how much you like what ARC automatically does.
Something to be aware of is that when you first listen to your corrected system, it may sound as if the equalization has reduced overall bass. It is easier to hear peaks in response than dips. ARC does more than level peaks; it also levels dips. With the equalization turned on it may be immediately apparent that boominess is gone, but it may take longer to notice that a wide range of bass notes are now audible, and for this reason, you might want to spend a week getting used to the new sound. Once becoming accustomed to tight bass with the entire frequency range playing at an equal level, chances are you’ll never want to go back to bloated one-note bass. If comparing ARC on vs ARC off, note that the subwoofer’s level is calibrated to match the other speakers when ARC is on. If ARC is disabled, the subwoofer level may require manual adjustment.
Viewing graphs for the first time may create the temptation to change targets immediately. There is rarely a good technical reason to do so. If you are not satisfied with the initial results, examine the pre-correction measurement curves. They show how your system performed before room correction. Does the unaltered response reflect the general performance expected from your speakers, especially in the bass? If not, do not try to compensate through room correction. It bears repeating that ARC is not meant to be a substitute for proper speaker positioning, nor can it reliably force your speakers to do something they weren’t designed to do. The purpose of ARC is to take performance to a higher level after implementing traditional setup practices.
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Finally found this forum after searching AVM70 in google.
some forum said the DAC chip going to be replaced with ESS DAC, does anyone know about the information?
I am interested on the MRX 1140/AVM 70. 
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Welcome bro. And Yes, I’ve read that new batches of the Anthem AVM/MRX will come with ESS DAC instead but to date, there is no confirmation from new customers showing the ESS chipset. Regardless of whether AKM or Sabre chip is used for the new batch, you will take comfort to know that the sound signature of Anthem will make you smile. 
Thanks, still contemplating should go for MRX1140 or AVM70. As i am will be either using 2.0 or 2.1 channel only since it is to replace my current emotiva xmc-1 in my room for music and movie. I am looking at AVR as i need a few hdmi input (ps4, nintendo switch, tv box) , unless i can find integrated amplifier that has 2.1 channel with hdmi input like bryston br-20 but price is overstretch (10k).
called to checked on the availability, seems like need to wait like 90-120 days for it.
You called who or which shop? The 90 days - 120 days sounded like E77 to me - i.e. the same dealer that helped me to get the first MRX 1140 into Singapore. Usually, you don’t have to wait that long as all backlog has been cleared AFAIK. You can ask Arvind to place an order for you with 50% deposit.
Yes, i called E77 to ask about it.
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dont think liao…just ordered an AVM70…lead time is still 90-120 days thou since last batch only reached last month. 
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Steady lar…
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Though I already have mine,… the room is not yet ready and my Focal in-walls… no news from Abs Sound…
So I think by the time I can appreciate its quality fully you would have yours on hand
hope it will come sooner…aiyah…i am just itchy backside want to change something. Hopefully will be better