mix and match subwoofers

A friend has offered to sell me his SVS subwoofer and I'm thinking about putting it in my home theatre with my existing sub. I know subs are not timbre or voice matched so I was wondering if there will be a problem. My current sub is a MKIV 15" Earthquake. Has anyone else tried mixing brands of subs into their theatre? I'm guessing it will take some work to adjust them correctly but I don't want to do it if the sound won't been seamless. Any advice?