There is always the risk of shock, and with tubes there can be hundreds of volts lurking about. Has nothing to do whether it's an amplifier or a CD player, high voltages are just as lethal in either.
The only way to know for sure is to use a digital multi meter to test the tops of the capacitors for residual voltage. Those silver relief covers on top are conductive and can hold voltage for quite some time. And as russ69 states above, reach in with only one hand. This way you wont be grounded if your other hand touches the chassis, greatly reducing the chances of being zapped.
Then again, if the back plate says no user serviceable parts inside, do not open, or risk of electrical shock, then you are not going to get an answer from the manufacturer. Lawyers and stuff.