one thing that i didnt understand when i first started in car audio was that you're not giving your sub the ohm load, you're giving it to your amp. it depends on the rating of the sub that will give the amp the ohm load.
say you have a dual voice coil sub. when you wire them in parallel, you're going to give it have of it's rated ohm load (2 ohm sub would be a 1 ohm load) and if you wire it in series, you're going to double the ohm load (2 ohm sub would be a 4 ohm load) so if you have an amp that's rated 1000 watts @ 1 ohm, getting a 1000 watt dual voice coil 2 ohm sub would match that amp.
also the more ohms you have, the more resistance you have giving you less power.
running amps at higher ohm loads also enables them to run cooler, making the signal cleaner and more efficient