Take 30 of these : http://www.parts-express.com/pe/showdetl.cfm?Partnumber=299-028
I am sure you have room for 3 on each chair base. That would give you 12 ohms per chair wired in series. Wire 3 chairs together in parallel and you are back down to 4 ohms.
6 chairs each with 3 shakers in series, would give you (2) 4 ohm loads. ( 3 chairs per channel )
2 amps such as the Inuke 3000 DSP would suffice, and give you the ability to eq up the bottom end of the shakers, since that model starts to fall off around 40 hz or so.
One amp would drive 6 chairs with 4 ohm loads per channel, and the other would run 4 chairs ( wire shakers in series for 12 ohms per chair, then parallel 2 chairs together for 6 ohm loads on each channel )
A call to Parts Express would confirm whether or not you would qualify for the 'large quantity discount'. 30 x 39bucks = $1170, two Inuke 3000 DSP's would set you back $ 800.... screws, wire and connectors possibly another $ 100. That's $ 2070 compared to $ 3500. The amps would have to be mounted away from the listening area though, as the fan is loud when they ramp up. The built in DSP is very easy to navigate.
That could free up funds for a nice 21" in a proper enclosure with ample power.