specific grille clothe can be bought online. via google... or honestly any "porous" cloth from spotlight will do ....
a test some one taught me was to stretch out the cloth and talk through it....and ask your friend if there is a difference in volume/tone...etc...
cus you dont want anything that will hinder the sound projected from the speakers. Well thats for those hi-fi enthusiasts with Golden Bat Ears"...so wont know if it will be significant enough to apply to guitar cabinets... ive been putting off refinishing my 5150 and modding it..so i may be getting some "spotlight" on my amp too...