The MAX setting does not work well with room correction and calibration. Actually it sounds horrible during these. It is definitely causing an issue. Set all sizes to zero temporarily for calibration and and MRC.
After the MRC is done, and saved, go back and adjust each to max size and store the settings. This will get you to where you need to be. The MAX setting is killing the SNR.
I'm not sure I have ever seen this advice before. I have had a lot of trouble with S/N on MRC and if this helps I will give it another go. I thought the speaker size setting only limited the bass in extreme circumstances and basically did nothing except in "overdrive".