These are exellent sub, this couple perfectly in stack of 4 per side is real happiness, against one per side is really disappointing. Their uses were not very easy but if we did and we quickly understand how well used. They are not very expensive but require real amp and true method, therefore it is expensive to use.
I heard a lot of sub / bass and have also used many, but for those seeking sub (without much punch, but very heavy) will be served and even filled.
These are sub EAW difficult (almost impossible) to use with the heads of another brand, but to complement it's heavy KF 650/750/850.