Amps designed with line-level inputs work just fine connected that way. No company would provide a set-up like that if it didn't work. You just have to be sure you are tapping the correct wires, the same sort of care that any installation of any sort requires.

I set up my Alpine sub amp this way and it is fine.

Now, in your case the entire audio system is evidently being powered by an aftermart amp and you think it is only louder, not better. I think that with the stock speakers that is all you could expect. They aren't very good and a million watts isn't going to make them better.

Another issue to consider is that there are a million wires back there connected via that fancy plug. Are they all tapped correctly? That is a lot harder than just hooking up a sub amp! There is a huge possibility of error unless a really knowledgeable person set this up.

In addition to that, I have no idea of how the frequencies are crossed over or blocked or not. You would have to know how the head unit controls the amp as well. Just powering up the wires might not work as intended because it might bypass all this and mess it up.

An X with DSP would complicate matters even more.

BSW is introducing a specialty amp soon I believe. It will be very interesting to see how they solved all these problems.

If I was forced to do a serious X5 audio upgrade I would replace everything, starting with a modern aftermart head unit, amps, speakers and wiring just to take anything BMW out of the equation and enter the realm of the known stereo world. There is at least one local X where a shop did just that. It was a model without the nav system or dsp.

As for the E70s...lots more wiring, and a far more complex interface. I would be very afraid to tackle that one beyond maybe better speakers.