If you don't want to change the files themselves with AACGain, which can be undone, than try a ReplayGain scan with foobar2000 and copy the values for the albumgain section to the tag part of the soundcheck.
Sound CheckSound Check is a volume normalization algorithm built into iTunes and iPod. Sound Check normalizes based on the loudest audio sample in each track. iGain uses the Replaygain algorithm to normalize based on the average perceived volume of each track. For this reason we believe iGain produces superior results. iTunes keeps track of the maximum volume and Sound Check adjustment in its proprietary iTunNorm file tag. iGain does not read or write this tag. If you enable Sound Check on songs that have been processed by iGain, they will not play back at the expected volume.For this reason, always disable Sound Check, both in iTunes and iPod, when using iGain.
On the Mac, iVolume uses replaygain to calculate either track or album values, and then applies those values to the ITUNESNORM tag so that it works exactly as iTunes and iPods expect.
EDIT2: To check your work, check and write down the replaygain value for a couple of tracks, open Itunes, make sure Soundcheck is turned on, play one of the converted files, right click>Get Info>Volume should match the Replaygain value you wrote down.
...EDIT: However, if these AACs have already been run through Itunes and had soundcheck values written, then those tags will have to be deleted in MP3Tag first, then run the above procudure. Load up all the AACs in MP3Tag>select all>right click>extended tags>highlight ITUNESNORM TAG>click red x to delete...