I think it all depends on whether the cdp was designed to drive the input section of an amp. DCS, Audio Aero, Wadia have up to 6 volts of output.(I've had all three). I can see where a typical cd player with an off the shelf out-put op-amp at two volts is going to have problems. The quality of the volume control also is important at such low voltage.
I had a Placette passive volume control woth a Marantz cd player and it was great...but only at 75% volume and up. the rest of the volume range had the dynamics problems mentioned here. I just think if you took the money it costs for a good preamp and put it in the player it is a better value...assuming youa re cd only. Good luck.