At the song selection screen, step on Left, Up, Right to play in Hidden Mode. In this mode, the scrolling arrows in the songs become invisible before they reach the top.
When you have Hidden mode selected, enter the code again (still on the song selection screen) to enter Sudden Mode. In this mode, the scrolling arrows in the songs are invisible until they are almost at the top.
When you have Stealth Mode selected, enter the code again (still on the song selection screen to enter Stealth Mode. In this mode, the scrolling arrows in the songs are ALWAYS invisible, forcing you to either memorize the song's steps, or look off of a partner's arrows.
When you have Stealh Mode selected, entering the code again will put you back to normal mode.
Kakumei, a collaboration song between Naoki and DJ Taka (prolific songwriters of DDR and Beatmania, respectively), is another secret song, obtained by AA'ing MaxX Unlimited (as a secret stage, not from regular song selection).
Kakumei is played on Reverse, 3x speed, and Sudden Death (one Good, Boo, Miss, or N.G. will automatically fail you!), thus to pass Kakumei you must get a full-combo. Practice your gallops, because this song is full of them.
To reach the extra stage MaxX Unlimited, 'simply' AA your final song (Get a Dance Point Rating of 92.0% or higher.) However, it must be on Heavy, and Little cannot be on (Although any other modifier, including Freeze-Off, can be.) MaxX Unlimited EX is played with 1.5x Speed, Reverse Scroll, Dark Mode, and Pressure Mode ON.