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

Can't you install a wall shelf....doesn't necessarily need to be an audiophile shelf like a Target, but I would think any sturdy MDF wall shelf would be a preferable alternative to putting the equipment on top of a fridge or subwoofer.

Good luck.

You can try isolating the equipment from vibration, using thick felt or isolation feet, but I would think your CD player would be more prone to skip at high levels. Also, depending on the quality of the subwoofer cabinet, some panel resonances might find sympathetic parters within your amplifier or CD player.
(buzzing from the sheet metal boxes, screws becoming loose etc...)

Generally speaking, I seriously doubt that your electronics were designed or tested for this type of placement. Crossover networks seem like a good comparison, but they are in fact designed to withstand vibration.

Let's see... a wall shelf above the sub?! Wished I'd thought to mention that before we had to endure all this unrelated crap. What's the big deal? Eighthcircuit, you need to conserve on floor space, I got that part of it. Could the sub vibration be an issue? Hmmmm... Possibly. Why not just keep from having any/most possible problems by decoupling the electronics from the sub by way of an auxiliary mounting? Good luck. Enjoy!

Buy a table (with four legs, I know obvious) that will go on top of your subwoofer (and around it), then put it on top of that. It won't touch your subwoofer and until you get a rack to put it on, you'll be better off than having it on a sub.