hi all new to the site.....right now I have a older (basic black and white)hummingbird 550 fish finder I want to up grade, looking at 581 I HD(black and white) sonar / gps(do I need a gps...for maps...don't no) and a 576C HD DI sonar color (NO GPS) (I think I want the down imaging) . right now I have a shoot thru ducer with the 550. if I up grade to one if these and it has a transom ducer can I install and use it as a shoot thru ? I really like the color screen and probably see thing a little better. now since these are all 500 series are the heads interchangeable with my 550 existing wires? what do you all think ,did you have to think about this before buying? glenn