You can see the operating temperatures on the sales page, I think sunlight wouldn’t be an issue over time. Given that the speaker itself is on the ‘inside’ of the device, though…it seems unlikely to have a significant affect. 

I recommend rotating the SUB such that direct sunlight will not hit the speaker cones. Eventually, the sun will damage the finish.

Maybe something like this will help: https://soundskins.com/product-category/sub/

You can make your own speaker cover out of standard speaker grill cloth too, much cheaper than the Sound Skins.

I’d use a light color to reflect as much sun as possible.

You might also look at using a layer of Reflectrix (foil and bubble-wrap) insulation under the cover. It is inexpensive and easy to cut to shape.

Simple answer is avoid direct sunlight. A more complicated one would involve other variables such as the time of the day/year the sunlight falls on it, how much time does it do so, does the sunlight hit the cones etc etc. And, in all those cases, use the suggested covers as long as the sunlight also does not heat the room up as well.