No covering is acoustically transparent. It's physics. However, you can cover them with something that may be mostly transparent.
If you were to drape something over them, it would need to be something like sheer curtain material, but depending on the overall color of the speaker, you may still see them.
The thing is, this may be a losing battle for you if you don't make some kind of compromise. You have to ask her what is acceptable for her and you have to state what you want as well. One method to reveal to her the reason for nicer speakers would be to play music she likes to listen to before you go to make a compromise. And at that time, if you two ever dance together, that would be a good time to sweep her off her feet to one of her favorite songs. Then talk about the situation.
I know that isn't really what you were asking, but if you are to keep the speakers in that room, you both are going to have to meet 1/2 way. Oh, and stop playing xbox or watching sports with the sound cranked !