OK, so we know that unchecking AAC VBR in Preferences > Advanced > Importing actually gives you ABR (Average Bit Rate) encoding and checking it gives you VBR_Constrained. I just put up a script that uses afconvert to import CD tracks as old-style CBR (Constant Bit Rate). However, even this apparently does not provide a constant bit rate. For example, after ripping a few tracks with it and attempting to join and export them in QuickTime, I do not get the pass through option. Checking their info with the QT Inspector the bit rates are indeed slightly different. Here's what's weird: CD tracks I ripped with a pre-v7.5 iTunes without VBR also have slightly different bit rates, but QT allows the export as pass through. I've gotta do some research.