Actually, I think the PXE-H650 may be bigger than I want. On further investigation, I am looking at the Blaupunkt THA-555.
This is a "plug and play" amp and simply boots the current signals to the speakers.
What I would really like is a simple device that removes all the filtering/equalisation from the speaker feeds.
I also saw this thread:
http://www.e90post.com/forums/showthread.php?t=157820
Where the owner simply got the tweeters from the upgraded stereo and plugged them into the midrange. Anyone got any thoughts on that?