How close preamp and CD player

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.