There are 6 Power Decks that you unlock by beating the Guest DJ) at each venue in Empire Mode in a Checkpoint Battle. (2nd to last option on the menu)
These are the decks and their abilities:
Tapping Deck - More points for tapping
Scratching Deck - More points for scratching
Multiplier Deck - Gives a max multiplier of x5, with a double max value of x10
Euphoria Deck - Euphoria lasts twice as long
Rewind Deck - Rewinds go back twice as far
Hypermode Deck - You only get half the sightline to see your notes.
You can beat the Guest DJ's on any difficulty level to receive these Power Decks, so drop the difficulty if your having a problem with any particular DJ. Also take note that unless you unlock the Venue in Empire Mode you won't have access to the DJ Battle in that venue