I use 4 shakers in my recliner loveseat- they really provide seat rocking effect during action scenes, but i prefer to turn the amp for them off when i listen to music.. they are good to have feature in your system definetly. I bought them from partsexpress - total cost with sub amp was around 200 bucks(US). Good Luck!
Wouldn't they be more effective using an LFE signal? My guess is that the surround channels do not receive a sufficient amount of LFE to really get a good result from the bass shakers. I don't use them though, so I am most like wrong.
Well I am using an LFE signal. My receiver allows me to use the 6th and 7th channels (surround backs) as a multi room application.
I run an LFE signal to an unused input (video 3 or something) and select that input for the multi room source. So all I do is turn the multi-room input on, and voila I have a whole lotta shakin goin on