Of what quality is the variable output device on the CD? Usually they are around the same quality/sound as a Radio Shack Receiver! Also the variable out may or may not have the ability to drive your amp. It may sound "ok" but you'd have to try it to see if it meets your requirements. But if sound IS ok then no reason not to.
A passive (high quality) volume control is another option from the full output jacks on your CD.
A passive (high quality) volume control is another option from the full output jacks on your CD.