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?
I am not familiar with the technicals of those subs,but I have had two different brands of subs in my theater before and one was a ported design,the other a sealed design and if the subs were on the same wall,you could pick out the ported one.They ended up in opposite corners in my room and if you were in the right spot(bad spot?)and you listened for it,you could still pick out the ported sub but it was minimal compared to when they were on the same wall.Just some of my experience.

if you do this, i would be interesting in knowing how the 2 subs compare as i also have the earthquake mk14 15. thanks.

aloha keith