How close preamp and CD player

If it were me I would have them next to each other or above below each other in the rack. Hell the closer they are then the shorter the IC run which means saving money :)

It's most convenient for me to have the CD player on top shelf, preamp right below, just didn't know if there was any reason not to. Thanks.

That is the way mine is setup.

I have used these exact units together, with the CD on the top shelf and pre just below. It worked perfectly. I don't think you should be concerned about any noise somehow being generated between the units. More important is ensuring there is adequate ventilation around and above the pre. I suggest at least 3" for each side and the top, and an open front and back. If you have less clearance, you should consider a small 12v fan. I run them at 6-9v to reduce the fan speed and thus the noise. Just find a cheap wall wart with the right voltage.

I have CD3MkII(top shelf of SolidSteel 5.4) and LS-25MkI(shelf below). No problems at all.

P.S.
Are you running balanced or SE from CD3 to LS?

I'm running balanced Discovery Essence. Thanks for the responses.

I have about 2 1/2 inches on top, open on all 4 sides.

that should do

CD players and D/A converters put out a large electromechical field, mostly in a vertical manner, that can interact with other components. I would recommend putting the preamp to the side of the CD player rather than directly above or below.

>>CD players and D/A converters put out a large electromechical field, mostly in a vertical manner, that can interact with other components<<

If the cd player (or transport or d/a) is well engineered and built, which includes proper shielding, proximity to other components should not be an issue.

I'm sorry; I meant to say "electromagnetic" field. DACs are especially notorious for emitting a large electromagnetic field that interacts with other components that are nearby; the electromagnetic field usually proceeds more in a vertical manner rather than in a horizonical manner so it affects components that are either directly above or below the DAC. If you don't believe me try taking a transistor radio that is pulling in a strong station to within a few feet of a DAC or CD player and see what happens. I've heard the improvement in sound by moving DACs and CD players well away from other components.