To find a subs motor strength you take the bl square it then divide by the re. When taking t/s specs on a dual 2 if wired in series the bl number will be a lot higher than if taken in parallel but the re in parallel is only 1 rather than 4 so the bl number alone can be very misleading. If A dual 1 and a dual 2 both wired in series show the same bl number the dual 1 will have twice as strong motor force. Weak. Motors tend to do well in sealed boxes and strong motors in ported. As far as actual output though there are a lot more things to take into account.