Check your connections on either end of the amp power wire too make sure you are not touching the car's body somewhere. If you see nothing wrong, then (small possibility) you could have cut the insulation when installing.
Also, check the wire that gets energized to turn the amp "on" too; not sure if it would do this but you never know.
Make sure the replacement fuse is the correct amp rating, not too far below and absolutely not above the recommended rating.