Reading some recent posts I've begun to doubt my current setup. I have a 2X10 and 1X15 currently. I only play 4-string basses and don't see myself changing in the forseeable future. Does the 15" add anything to my sound or would I be better off with a 4X10. I am refering specifically to frequencies and the like. Does a 4-string bass hit on any frequencies that a 15" can provide and a 10" can not?