Bit late to this topic, but I have 2 50w AuraSound AST-2B-4's screwed directly into the bottom of an IKEA Markus chair.
One 50w shaker is easily enough for most 'direct-to-chair' setups, as long as you're able to mount it near the centre line. This wasn't a possibility for me, so when I had just one wired up, the vibration was biased to one side. I added the second one to balance it out.
I usually run them at 50% volume then dial the simshaker settings in at between 25 - 75 depending on the effect. Any more than that and my neighbours would probably complain.