15W amp is a bit underpowered for what you are attempting to do. Outdoors is suprisingly noisier than indoors (moreso for me, since I live next to a busy thoroughfare).

Although your gear will be residing in an enclosed area, it's not protected from humidity...outdoor speakers are sealed from those issues, but the amp, not so much.

Otherwise, your setup sounds fine...but you will need a more powerful amp. 2-channel units aren't very expensive these days.

__________________
2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car