I picked up Amplitude a while back, and while there are some aspects of it I loved, I think I prefer Frequency overall. While there are several songs in Amplitude that I like, there's far more that I find irritating compared to the Frequency soundtrack (where there were maybe two songs that I just can't stand to play).
And while I like how the score doubler power up can be handy in giving your score a boost, I don't really care for the way scoring is handled with the freestyler. It just gives WAY too many points, making getting a high score on songs more of a matter of lucking out with the powerups instead of trying to do better on the song itself.
The thing that bothers me most has to be the general playfield changes. I absolutely hate the flat layout from a gameplay standpoint. Sure, it doesn't feel so closed in, but it makes the game unnecessarily hard at times. In Frequency, if you had to jump over three tracks in a hurry, you could see what notes you'd have to play by looking ahead on that track, which would be somewhere above you, but still clearly visible. In Amplitude, the track you need is off the side of the screen, requiring you to pretty much memorize the song in order to continue your streak. Not only that, but the stupid arrows that tell you where you have to go to continue your streak block your view of the notes until you're about to start playing, resulting in even more irrritation.