Sub Question. One higher quality sub or two lesser quality? (Rythmik vs HSU)

I would NOT add subs to those Cornwalls, with 15" woofers, ports, and high efficiency horns.

You could easily mess up what you have, it's very hard to add just the lowest ...... down to ....

You are looking at specs that say down to 34hz. that is in a test space, open air, your's are in a room, sound waves contained, reflected, your bass is maintained below 34 I suspect.

I strongly suggest you buy a decent sound meter, see what you are actually getting, and where. Look for any weak/too strong spots, standing or cancelling waves.

With that sound meter, if you add subs, (or change speaker placement) you will be able to measure/adjust the crossover/blend from your listening spot.

2 channel system: 1 pair of subs, directional, located next to the cornwalls, forward facing. Bass is stereo, and the overtones of all low frequencies also give directional cues.

      Yes, I use the Dayton SA-1000, that Duke mentioned, in my AK Debra 4-sub DBA.  It's been on virtually 24/7 in my system for over 5 yrs now and still is performing flawlessly. 
     A good option is to buy one of these for $300-400 from Parts Express or Monoprice and build or buy 4 passive 4 ohm subs and create your own custom swarm. The Dayton also functions as a sort of sub control unit since it has controls for volume, crossover frequency and phase along with l+r ch line level inputs and A+B sets of speaker outputs.  Its 1,000 watt class AB amp has plenty of power to drive four 4 ohm subs and its controls allows for the setting of all four subs together as a group.
   Another method many use is the MiniDSP,  which has a single set of analog L+R line level inputs and up to 4 L+R analog line level outputs to connect up to 4 self-amplified subs.  This unit also has settings for volume, crossover frequency and phase plus the capacity to customize the crossover filter slopes.
    There's many ways to skin a cat.  No matter how you do it, however, the cat can be relied upon to protest vehemently and loudly.

I've never skinned a cat as far as you guys know,
Tim

I see two speaker outputs on the Dayton. How does that work to drive 4 subs?

2 on each wired in series?

mapman:
" I see two speaker outputs on the Dayton.  How does that work to drive 4 subs?

Hello mapman,

     It's actually 2 pairs of speaker outputs on the Dayton sub amp.  I have a front pair of subs connected to the top pair of outputs and the other pair of subs connected to the bottom pair of outputs.
     The connection method of the AK Swarm and Debra systems' subs are also a bit unique as all 4 subs are connected in series/parallel  configuration.

Tim 
    

As I read it the two outputs are already in parallel and connecting two subs to each directly would result in 4 in parallel which would could result in loads 2 ohms or less with nominal 8 ohm subs. Two in series to each would seem like a better idea?