I currently own the JL H.O box with a 15" circle cutout for the subwoofer.
i am interested in purchasing a kicker l3 square subwoofer in replace of my current subwoofer. Would it effect or harm the SQL or SPL by cutting the subwoofer box to fit a square sub?

 

The potential drawback of doing this is not the cutting of the hole from round to square but more putting a woofer into a box that it may not want to work in.

Double check that your Kicker will work with that box.

 

bad suggestion each sub have different specs

 

IF the sub perameter fit within the spec of the box and IF the box is well made, go for it. If not, a sheet of wood is in your near future. It's not hard, or all that time consuming.