You can easily A/B whether the ROOM CORRECTION part of the processing is engaged but you can not easily A/B the changes ARC made in cross overs and speaker levels as part of its setup, nor the fact that using ARC disengages other parts of the D2's audio processing that you might have used in a manual setup -- such as the Room Resonance Filter.
As has been noted here, ARC tends to overlap cross overs, which presumably makes it easier for them to implement room correction in the range of the cross over frequencies as cuts in volume -- which are easier on the system than trying to boost dips. I.e., the overlap produces hotter audio in the vicinity of the cross over so more of the things that need to be corrected are peaks instead of dips.
And THAT means that if you simply turn Room EQ = OFF you will be getting results that are biased by the overlapped cross over settings and the particular speaker levels that ARC used -- i.e., without the benefit of the compensating cuts the room correction processing would have applied. This may very well make the audio sound worse than it could with Room EQ = OFF and a properly detailed manual setup.
You could do the following:
1) Do a manual audio calibration to get the best you can get by other means as regards speaker levels, speaker cross overs, Room Resonance Filter, Center EQ, speaker distances and subwoofer Polarity and Phase and whether or not it is THX Ultra2 with Boundary Gain Compensation needed. You will need the speaker distances and subwoofer Polarity and Phase for use with ARC anyway so that part is not additional work. WRITE DOWN YOUR SETTINGS.
2) Run ARC and tell it you want your Movie and Music configuration to be the same. Upload those ARC results.
3) Set up a Source to use the Movie configuration and to have Room EQ = ON. That will make it use the cross overs, levels, and room correction parameters ARC calculates, plus the speaker distances and subwoofer Polarity and Phase that you set up manually. The Room Resonance Filter, Center EQ, and subwoofer THX Ultra 2 and Boundary Gain Compensation settings will be ignored.
4) Go into Setup / Speaker Configuration and change the Music Configuration to be different from the Movie Configuration, but using all the same speakers. Manually enter the cross over values that you wrote down from step 1 into your Music Configuration. [LEAVE THE MOVIE CONFIGURATION UNCHANGED!] Also enter your Room Resonance Filter, Center EQ, and subwoofer Polarity/Phase/THXUltra2/BoundaryGainCompensation settings as necessary, i.e., if they are not already in your Music Configuration. Your speaker distance settings are shared with the Movie Configuration so there's no change needed there.
5) Set up another Source to use the Music configuration and to have Room EQ = OFF. That will disengage the ARC room correction processing and will re-engage the Room Resonance Filter, Center EQ, and subwoofer THX Ultra 2 and Boundary Gain Compensation. In addition, the cross overs will be the ones you just set manually.
6) Now go into Setup / Speaker Calibration. The levels in there are the ones set by the Upload of ARC results. There is no mechanism to provide distinct levels for Movie and Music configurations. Compare the levels that ARC set to the ones you wrote down from your manual setup in step (1) above. If they are different, measure the levels ARC set up with your SPL meter to see how well they will work with your Music configuration. If they appear out of whack, your only choice for a true comparison will be to change them manually when doing the A/B. You MIGHT be able to avoid this by using the "temporary" speaker level adjustments (via the remote control) which are remembered separately for each Source, but this may not produce precisely the same results as changing the settings in Setup / Speaker Calibration.
7) Make sure both of these Sources are set to use the same audio processing mode for the test content you will be playing (i.e., PLIIx or THX or whatever). Now you can A/B by switching between the Source you have set up to use Movie configuration with Room EQ = ON and the Source you have set up to use Music configuration with Room EQ = OFF. (And as stated in step (6), you may also need to adjust speaker levels between them.)
There is, of course, a much easier approach presuming you are willing to forego instant gratification:
A) Do *NOT* run ARC yet. Set up your D2 to produce the best audio you can by whatever means you have available other than ARC.
B) Listen to your system for, oh several months. Constantly refining your audio setup as necessary to get the best you can out of the system. Be serious here. Try to set it up right.
C) After you are well familiar with how that sounds, *NOW* run ARC. If your first 15 minutes of listening after running ARC doesn't convince you ARC is the cat's pajamas then there is something wrong in the way you did your ARC measurements or upload, or you forgot to turn Room EQ = ON for your Sources.
I mean really. For the folks here who haven't had problems getting ARC to run correctly -- which is by far the majority of the posters here -- the wildly positive comments are far and away the norm.