Grill's on or off? It boils down to how the designer miked the speaker in a anechoic chamber to achieve a perfectly flat response. Some designer's will mike and adjust the crossover with the grills on and some with the grills off.
Call the speaker company you plan on buying from and ask the engineer's if their speakers are adjusted in a chamber with the grills off or on. This is why some speakers sound better with the grills off and others with the grills on.