OK, so I tried out iVolume this morning. It's shareware, of the nagware variety, so after you process maybe 200 songs it starts halting and prompting you to register every 25 songs. After 1000 songs it stops altogether.
It is super easy to use, and seems very fast (when it's not paused). I haven't tried listening to the adjusted songs yet, to see how the leveling worked out. Maybe it will be worth the $30, I don't know.
I think I'm going to give aacgain a shot, too.