Grill impact on LF

I can't find much concrete info on this so anything anyone can say to help is most appreciated. 

We have two 18" PD1850s loaded in two boxes. At the front there is an opening big enough for someone to put a hand in and damage a cone. Or drop a can in there or similar. 

We'd like to cover the gap with grills. Gold ones that look sweet. 

The grills have a 65% opening, and i'm told that is the requirement to not interfere with the frequencies. However, i've also heard the they will still be effected at higher power. 

Is it correct that at high power there is a drop as turbulence adds more resistance?

I read somewhere that grills only affect higher frequencies and I shouldn't worry. 

Someone else mentioned that the bass will be affected but I shouldn't worry because it's not noticeable, and, because we have a bit of head room from the amps, any loss can be compensated for by turning it up. 

From the perspective of protection and appearance we'd like to add the grills but we'd like to know if there is a negative to doing this. 

A not very loud ear test proved inconclusive. However, being in the city, with our resources, it isn't very easy to do these kinds of tests. And so, here I am. 

Thanks for reading. We really appreciate your time. 

Hank