Each amplification stage, capacitor and cable add distortions and noise.All those gain stages are in an integrated amp too. Usually though you have a greater chance of lower noise with separates since crosstalk and intermodulations from the other channel won't exist.
To see how profound this is, this morning I was working on an integrated amp, and noticed a very distorted signal at the input of the power amp- on the first stage and throughout that channel. Chased it back to the input, which is all passive components on that amp, and realized that since I was only driving one channel, the unused channel had that distorted input (and output) going on as a result. There is nothing malfunctioning about this amp; but apparently this sort of signal (the result of crosstalk) is adding distortion to the amplifier, which it would not have were the channels merely separated. The amplifier was an older solid state amp made by Trio (Kenwood/Pioneer).