Cycling with a large chunk of raw shrimp is the safest and most effective in my opinion..
The rotting shrimp will create an extremely high amount of ammonia, growing a large amount of nitrifying bacteria. I prefer the bacteria additive 'Stability' by Seachem, as I have cycled large ray ponds in just a short period of time with a combination like this.
Using an existing filter will work well also, but keep in mind that you only have as much nitrifying bacteria as your bio-load. Your currently cycled Xp2 only has as much bacteria as the amount of ammonia that has been going into that tank. So unless your 75g is extremely over-crowded, the current bacteria being harbored in the biomedia is only 1/10th of what you will need for Rays.
Rays have an extremely large bio-load, thats why overdoing it with the Ammonia spike at the initial cycling of the tank is important.
Also, make sure you have enough surface area to harbor the large amount of nitrifying bacteria that you will need for the Rays.. I often sacrifice the area in canister filters that is designated for carbon, too add more biological media, such as BioMax Beads or Matrix media by Seachem.
You also might look into a wet/dry trickle filter, they are very low maintenance and are excellent at harboring large amounts of nitrifying bacteria.