Hi there... serious computer geek here. We're talking 14 years, 2 degrees, 10+ certifications, and a myriad of positions, the present one being a regional IT manager for one of the largest engineering firms in the US.

/dismount high horse

I bragged about who I am just so I could hopefully put your mind to rest about this little question you have and you'll believe that it's an authoritative answer.

For your hard drive to incur electromagnetic damage from your cabinet speakers, the magnet of the speaker would have to be directly on top of the hard drive. Not the case of the laptop where the hard drive is, DIRECTLY on top of the hard drive. Even then, with the hard drive sitting exposed directly on top of the magnet, it's not a guaranteed that the hard drive is going to suffer damage. Even when we destroy hard drives, we have a huge magnet that we set on top of the drive and THEN we hit it with an electro magnetic pulse that basically turns the magnetic field on and off rapidly so that the plates inside the hard drive’s casing pull together and cause permanent damage. If you can put a piece of metal on top of the amp or cabinet and move it with no resistance from the magnet, don’t worry about it.

Finally, I put my laptop on top of my amp all the time. If that’s not reason enough, I don’t know what to tell ya. Good luck.