Wow. Lots of questions there. I have used Aura Bass Shakers in the past, but you are right, there are several options. The Bass Shakers are more of a 1-per-seat type install.
I would look at the specs for the transducers and see what will work as far as load on the amp that you plan on using. You may find that leaving the Butt Kicker on the main couch and putting a different brand either on the chase or under the platform closer to that side may work and be a better match for price and load.
The trick with a chase is that since it is a large piece by itself, putting a Butt Kicker on it in one spot may not provide enough motion by the time it reaches the foot end of it. While not as critical, you don't want it to feel like the epicenter is on your back, and yet the rest of your body doesn't feel nearly as much.
That is where I think that something under the platform could work really well. It would help to distribute the total vibration across a larger surface, regardless of what is on that surface, be it a couch, chase, or folding chair for that matter.
You will lose a bit of vibration strength since you will need to build a beefy riser to support the furniture, but since it will be rigid, the vibrations will be "smoother" across the whole riser.
So if 1 Butt Kicker works for your couch to a level that you like now, then I would think that 2 would work well under your riser. You will just need to "turn them up" a little bit more than you have it now.
If you really want to go with a different brand, I think that i recall Clark (as you mentioned) being one of the first companies that advertised their transducer as being designed for underneath flooring. I just don't know the cost difference there for you.
