Those subs are definitely loud, and large, but they wont really have any response below 50hz.. From the sound of things you're running a HT with regular HT speakers, so... You really should be looking at subs suited for the task. Based on the $600 price of the Behringer, you definitely have subs capable of very good output, and response down to at least 25hz available.
Only catch is they all ship to North America and nowhere else.
Getting good gear in Australia is an expensive exercise!
I did have a look at some of the kits on parts express with the postage being as expensive as the units themselves, that said they still do seem to better value than alot of other ht gear available here
That definitely hurts the value of everything... Even so, something like this PE kit costing $600 with shipping is likely going to be better then what you have locally for the same, including those PA subs.