Actually, the ground plug is often considered the culprit for ground loop hums, and the most common solution is the "lift the ground" with a cheater plug. A cheater plug on a piece of equipment designed to use a ground is not the best solution from a safety standpoint, but a product designed to not require a ground plug is often less susceptible to the problem.

It may not be the most scientific guideline, but I look for the crossover that sounds best in my room, with my hardware, and to my ear. Try a couple different crossovers with movies and music that offer some deep punch to them and see what sounds best to you.