Nothing. Straight from CD player in the power amp, CD player has 2Vrms (0-VU) max. output, and I used volume function in it, adjustable in 2dB steps - in experiments phase.
Although CD through preamp/buffer can sound stronger, sharper, even cleaner and more detailed - IME, it`s a "sound effect", like additional EQ, or so. Nothing really wrong with preamps, but I don`t need them.