Depends on how you add them. If you buy another pair, then run two series of 3 shakers in parallel (for a 6 ohm load) and not 3 series of 2 shakers in parallel (2.67 ohm load) unless your amp is rated for a 2 ohm load. Going under ohm rating for the amp could cause it to over heat.
I have 10 shakers (3-4-3 = 4.4 ohms) running off a 350 Watt amp at 4 ohms (250 watts at 8 ohms). I have the x-over set to about 50hz (hard to exactly tell) and I guess the level is around 20% on the dial. There is a whole lot of shaking going on.
Since each of the shakers is 25 watt, you may be maxed out with your amp, but if you already have them, then I would give it a try. However, with shakers, I think the more headroom on your amp, the better.
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