Thinking I need a Sub...

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. Just move it around until it sounds right at your Sweet Spot. REL's idea of leaving your mains alone (high level input) simplifies things and keeps another thing out of the signal path to take advantage of your amp's tone, which is a good idea that actually works. Also, dealer opinions notwithstanding, RELs are great sounding and reliable...the models differ of course, but you can get great results from lots of 'em (I bought a used Q150e and it's better than a smaller new one I've listened to), and you can simply pay attention to the level setting to avoid using digital compression/limiting devices which are often unnecessary.

"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.

I agree with Karl_desch, so upgrade your speakers if you must improve your bass. There's a reason you must spend considerable bucks for a true full range. As Karl put it, "coherent balanced sound." Adding a sub may give you the low frequency you crave, but you may not get a perfectly smooth sound. If you can still find a high-end dealer with a store out there, try listening to a quality restricted LF speaker with a sub, and then listen to that brands full range entry as a comparison.

01-15-14: Kennythekey
I agree with Karl_desch, so upgrade your speakers if you must improve your bass. There's a reason you must spend considerable bucks for a true full range. As Karl put it, "coherent balanced sound." Adding a sub may give you the low frequency you crave, but you may not get a perfectly smooth sound.

Going full range "may not get a perfectly smooth sound" either, especially if you're trying to get that balance of smooth extended bass *and* optimum dispersion, soundstage, and imaging.

At least with a sub you can do several things about integrating the bass with optimally placed monitors--change the subs' gain level, change the crossover frequency, high-pass the mains or not, change the phase, and change the location. Sure, that's a lot of potential adjusting, but it also allows several bass integration options that are not possible with full-range speakers.