I think its fair to say that most manufacturers don't recommend that, but most people do it.
The issue is with allowing cooling air to circulate through the amp; you normally need to allow an inch or two above the amp.
Of course, you could put the amp on the player, but the player is usually smaller, lighter and nothing like as strong. And it looks a bit daft.
No good answer, I'm afraid, apart form a stand.
Nick