Mixing and matching subwoofers hardly ever gives you optimal results.
Agreed, but sometimes different types can play well with each other with no ill effects, but people MUST measure and confirm.
Very often, what people are trying to do with different subs is to "get the best of each one" and hoping they will add just fine.
But what they FORGET, is that they are ALSO getting the "worse" or the problems of each one ALONG with the "advantage".
It does not often work out the way they want or like like to think it does.
But every now and then you do get lucky