adjusting input volume?

Does anyone have a recommendation on adjusting the input volume of individual players?

When switching my stereo from the cd player to the computer sound card, the cd player is much, much louder. The cd player uses an optical connection, so I don't think there's much I can do there.

It seems logical that a preamp might be used to boost the level of the sound card (Sound Blaster Live). I worry about further dimishing the sound quality on the mp3 with a cheap preamp, but then I don't want to spend a bundle either.

I'd like to be able to have both cd cuts and mp3 files in the same playlist. CDJ is very cool in that it can switch the input of the stereo amp according to the player. A very nice feature, but the differences in volume are too great.