I think it depends on what you want to do with the signal. If you can find a full signal, you can power anything. If you only pull from the tweeter, its not going to be a good source for a sub, as it can't fill in areas that don't exist in the signal.
This video did a good job of explaining...
Not a bad price, and it looks like it comes with a DSP to level out mids and highs to go with the sub as well. Issue I have is that it is a dealer only install.