Volume level

Hi so at one point or another I did an ALT + a on my itunes music and adjusted all volume output to plus 100%! How do I default them to the same volume level as newly added songs without having to do it for each individual track? I can change it by right clicking a particular track and changing the setting in song info, however at the time I did this there where a few thousand tracks on my itunes. So going track by track is near impossible. Those now just sound distorted on most hifi systems due to the volume increase and would like to change that. Thanks for any help 🙂