He's right, the OEM BMW speakers do have some odd impedances, from everything I've read.
Bavsounds doesn't make the amp you referred to, they just resell for Audiotech Fischer. But at $1000 for the amp alone they can keep it. I'll go custom before I pay that. I think I read where it was specifically designed to handle to weird impedances of the speakers. Unless you replace the stock ones with the Stage One speakers then the aftermarket amp coupled with the Stage One speakers might be directly compatible, but I've never done it so I can't say for sure. Just going on the reading I've done. Because if you do the Stage One replacement speakers with the OEM amp they tell you to put the provided resistor in line when installing so it doesn't throw the amp all out of whack.
This is one of many Youtube vids by the Bavsounds folks. They have one that is so long they broke it up into 5 portions that goes through a complete install of Bavsounds Stage One speakers with one of these Audiotech Fischer amps. Bavsounds site says you can hook up a laptop and tune the amp to customize the sound, so it seems pretty versatile, but not $1k worth of versatile. That thing is for folks who want premium but don't want to mess with paying someone to do a custom install, and can't or don't want to do it themselves.
https://www.youtube.com/watch?v=HYtxZRUmUYs