Thinking I need a Sub...

If you do go the route of subs look into the new JL's (I've not heard them yet but do use the F113). I used to use REL's and had many, many issues so my recomendation is to stay away from REL. Just my 2 cents worth.

(Dealer disclaimer)

I have the Rel R218 mated with Cremona Ms, just the right amount of extension for my ears. My buddy has the T7, and the bass is not as refined.

There are good and bad to both solutions.

The larger speakers will allow a more full frequency response that may satisfy your needs. This depends on your room, your system and your preferred lessening level. The dual sub option would more likely meet your lower frequency needs but they may be more difficult to integrate into your system for a coherent balanced sound. You may end up spending even more to get the right software fix for bass integration.

I personally would go for the larger speaker if you like the NOLA sound. Then in a year or whatever, if you are still not satisfied, think about subs again.

No right or wrong here.

Jump on 2!!

I also like the SVS SB2000 sub recommendation, although I'd definitely go for two subs since this is a music-based system and bass will be more evenly distributed throughout your room. SVS runs a deal on dual subs, and you can get two SB2000s for $100 less than one REL R218, and the SB2000s go down to 19Hz (-3dB) and have a more powerful amp -- probably a lot better for your synth bass stuff among other things (much better than Contenders too). And SVS offers a 45-day no-risk in-home trial. Absolute no brainer IMHO.

No matter what sub(s) you get if you go that route, you'll need to get some kind of integration software/device. I believe Velodyne still offers a decent unit that's not too pricey and can at least get you in the ballpark, or maybe even better a DSSpeaker unit that may be possible to buy used? I think pairing the SVS subs with your Boxers and integrating them properly would be a revelation. Keep us posted and best of luck.