Disco 2 new head unit

OK, bought a new head unit to replace the Alpine tape player, the problem I have is the new head unit has it's own amp feeding the amp under the passsenger seat and after doing a lot of reading I've learnt this is not good!!

The problem I have is hissing when playing through a USB stick and the preset volume when I turn on the ignition nearly breaks the windows, now I'm guessing I should go without the factory amp but still want to use the sub and all the speakers in the car, any ideas/experience would be a great help.
Cheers
Chris

Yup just remove/bypass the existing amp or use the pre-amplified outputs of the new head unit, if it has them, for the door loudspeakers and sub.