Think we have discussed this in previous threads, I've listened to a couple of setups using those sound on top amps and they just didn't cut it in my opinion (No depth of sound and very poor range). The problem with using the line out on the original kit just means you are amplifing a bad source, you need to have good quality from the start before you start processing/amping it.
The sound on top is just a way of making the standard system the way it should be, I'm not recommending that to fox though as he clearly wants more. I mentioned those as I have one and it sounds better than an aftermarket radio with the standard BMW speaker so the potential is there.
With one of those Soundgate units he can fit whatever amp he wants to the standard head unit and they sound really good.