db electrical has good priced alternators as well as power bastards. Always do your research on a product you are considering. What size is that l7? Also what year is your expedition? 1st gen? Vehicle resonant frequency is around 44hz. 1500 watts is plenty for the expedition. I`d tell you what I have installed now but you would laugh. My buddies tell me it sounds loud (not really that loud) than I show them what I have and they laugh in amazement and say its damn loud for the power.
It`s ok but not loud enough for me and definitely not as loud as the 2 18`s I used to have. :| I miss being loud.