Unlock Max 300
You can unlock Max 300 in more ways the one. One method is to get an AAA ranking on any song. You can also unlock Max 300 by using the roulette until you land on it.

Unlock All Hidden Songs
Graduisic Cyber - Get an AA rank or any song
Colors (For Extreme) - Complete 90-100 songs
Healing Vision Angelic Mix - Get a C or higher on all three Healing Vision difficulties
Max300 - AAA any song or pass it on roulette
MSG2-Mission R - A any song
Orion.78 (Civilization Mix) - Complete All of Challenge Mode
Paranoia Rebirth - Get a C or higher on all three Paranoia Eternal difficulties
Put Your Faith in me (Saturday Night Mix) - Quit or Fail a song
Sana Morette Ne Ente BLT Style - Get a C or higher on all three Sana Morette Ne Ente difficulties

Faster Song Unlock
It is much easier to reach the various song unlocking quotas by simply selecting a song in game mode. Once the loading screen disappears, hold BACK to exit the song. This counts as a failure, but it will count as a played song towards the quota.