Thinking I need a Sub...

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.

"Low Bass doesn't care about its distribution in your room as it goes where it wants, so a single sub can work very well in all but the largest rooms."

Respectfully disagree, especially for music. Low bass might not care, but your room sure does as do your ears. All else equal two subs pressure the room more evenly and can better help even out peaks/valleys within the room (3 or 4 subs even moreso). And two subs will obviously be working a lot less hard than one. Then there's also the prospect for stereo bass, but let's not go there here. Suffice it to say I'd take two good subs over one very good sub all day if that's all the budget will allow.