X = High Flow/Low Head - these are quieter and less watt/gph, but head is a lot less
The Hammerhead and Barracuda are different pumps - different pressure, difference in noise
Mags being loud. You can run a Mag dry (externally) or internally, your title says "what.... external..." You go on to post that you want to use Mag 12 or 18, so, b/c of your title I assumed that you would be using the Mags EXTERNALLY, in which case, they always make an annoying noise.
No, you do not count the distance from Sump to Pump, flow must be entirely unobstructed when using these pumps, they are not suction pumps.