You'll definitely lose FM with that amp and the stock HU. It won't be completely gone but if your out in a rural area it ain't happening.
I can attest to this first hand. If you live in or near a city it still picks up, but cross the county line and it's gone.
One thing I did to regain some of my reception was get a set of old metal whip antennas off of a 2008 Ultra Classic and that brought some of the reception back.
If you've got the time to wait and the cash you might want to just get the better amp.
Sorry to resurrect an old thread but just came across this while searching for help on my install.
Just wanted to say that damned if you guys ain't something else!
My hat is off to you all.
Hoggly.