1. An apparatus for processing a multi-channel signal, wherein the multi-channel signal is represented by a block of spectral values each for at least two channels, comprising: a similarity determinator for determining a similarity between a first one of the two channels and a second one of the two channels, wherein the similarity determinator is formed to calculate a first prediction gain from a prediction of the block of the first channel and a second prediction gain from a prediction of the block of the second channel, or first reflection coefficients for a first prediction filter for the first channel and second reflection coefficients for a second prediction filter of the second channel, and to obtain the similarity using the first prediction gain and the second prediction gain or using the first reflection coefficients and the second reflection coefficients; a prediction filter processor performing a prediction filtering, wherein the prediction filter processor is formed to use a common prediction filter for the block of spectral values of the first channel and the block of spectral values of the second channel for performing the prediction filtering if a similarity is greater than a threshold similarity, or use two different prediction filters for performing the prediction filtering if the similarity is smaller than a threshold similarity.