what kind of subs are they? Isobaric requires one sub to be out of phase to complete the push-pull sequence. This setup allows the speakers to get more excursion and therefore possibly be using more power (even though you may not necessarily be hearing more sound) this added draw in power could be blowing your fuse. If everything is the same as before with the wiring and it worked fine before I would try upgrading the fuse. what size amp and power wire are you running?