two 10's or one larger subwoofer?

To get a good answer, I think you need to talk a little about what your system is and what you like and don't like about it. Otherwise, you will get a lot of random comments that will, most likely, be of no use to you.

I will give you a random answer. The frequency extension does not always correlate with driver size or "better" sounding bass. Just check the specs on the subs you are considering. I am sure that some will claim that the bottom octave is a must.
Personally I really don't find subs a good fit with my gear which doesn't claim to reproduce anything lower than 28 Hz. I like a clean, solid, punchy and dynamic bass. My system does this. Thus I am not interested in a bigger bass emphasis which I hear when most subs are integrated into the sound. Home theater is another matter where LFE are important.

I am aware that larger subwoofers typically have lower frequency response. Does that mean better bass?

Are you wanting more bass from what your speakers already have OR add bass that your speakers can't reach? Example I have monitors that are rated to 45hz (they do go deeper than the ratings) and I wanted to blend the lowest octaves without adding to what the monitors already can do. If I set my crossover at 32hz I just add the lowest octave(s). It blends in perfect with my monitors (I can't hear where the sub is located). Now some will say cross it over at 80hz and what you will do is add to what the speakers already provide and have the lowest octave(s) BUT the lowest octave(s) will not be proportional to mid and upper bass (which tends to let you hear where the sub is located). So which scenario is better? You have to answer that.

That being said larger drivers will go sub sonic and pressurize a room better. How low do you want to go? My Rel B3 is rated to 18hz @ -6db so I can only guess it goes to approx 25hz @ 0db.

Blk25 and Xti16

I am generally pleased with the sound of my system. The highs and mids are to my liking, yes they could be better but overall not a concern (FOR NOW). When I pull my speakers out from the front wall to increase the depth of the sound stage the low frequencies seem to fade fast. According to the manufacturer the frequency response is 30hz-28khz @ +/-3dB. Also they claim a low frequency limit of 27hz @ -10dB and should be placed no less than 12" off the front wall. They are 36.5" away now. I would like to cross the subwoofer(s) over as low as possible so my speakers can do what they were designed to do. What I want from my system is more QUANTITY of low frequencies but I am not willing to sacrifice QUALITY.

Tmega,

I wouldn't be so worried as to the size of an actual driver. If you want to make bass from a driver you can either, all things being equal, move a large driver relatively slow or move a small one very fast. What this means is that you can get the same low frequ. with different size drivers. There are many different, and valid, ways to achieve the same result. The reason I requested you list your system is just for this reason. If you are using some, cheap, mass market, receiver that only makes power on a spec sheet, I can absolutely guarantee that a new amp will be a much bigger upgrade than a sub. Like the cops say, you just better come clean and tell us everything.