Can I put receiver and CD player on top of my sub?

If it's such a bad idea because of vibration, then why do 99% of the loudspeakers in the market have crossover circuits inside the speaker cabinet? Placing an amp on top of a sub is really no different in concept than a self powered loudspeaker. A CD player is more problematic, but they all contain extensive error correction circuitry anyway. Also, depending on the construction of the subwoofer, the top panel may not have significant levels of vibration.

Is it a great idea, no, but is it workable, yes. And it won't damage the equipment.

"A CD player is more problematic, but they all contain extensive error correction circuitry anyway."

IMHO, a CD player on a sub is ENORMOUSLY problematic. Error correction or not, the sound of a CD player is degraded terribly by vibrations (though I seriously doubt the equipment would be damaged, at least over the short haul). You really shouldn't be investing in high end equipment if you are going to use this sort of set up. Severe vibration can degrade the greatest of digital sources to near Best Buy level of performance.

But, Eighthcircuit, don't feel bad for the question. It is perfectly appropriate to ask if you don't know. I myself was skeptical about all the fuss about vibrations until I demonstrated it in my own system. In my case I moved my system from the side of the room (where it was to the side of my speakers, supported by a beam running underneath the floor, and by a custom fitted cabinet wedged into an alcove in the drywall) to the center of the wall and in between my speakers. It was all motivated by a desire to buy high grade speaker cables in reasonable length runs. In making the move I even added a high quality rack, but it simply couldn't begin to make up for the change in support in the flooring and cabinet, and my system sounded broken in comparison. This was in a house that is actually quite well constructed. The change lasted less than a day, and I am now a firm believer in the perils of vibration in audio, and happily use cheaper cables in long runs.

I think some here are not being helpful, just answer this guys question.

Make a shelf unit that straddles your sub and provides a firm base for your components above.

Chad...okay, your guilt trip worked...you are right that my response was not "helpful" and that I should just stick to answering the question. I apologize to Eighthcircuit...But my initial impression was that this question sounded like a tongue-in-cheek question...(like whether I should rub vaseline on the outer jacket of interconnects to keep moisture from penetrating), but now that I re-read it, I realize that Eighthcircuit was asking a sincere question, and deserves a meaningful response.

So, my opinion is in line with all others...that putting your amp and especially your source on top of a sub is not a good idea, because the vibrations, while not harming your equipment, will make your system sound like a common car stereo.