Bass Shaker setup not working?

[GAWD]

I got two Aura Bass Shaker from Parts Express:

Aura part number AST-2B-4.
*Power handling: 50 watts RMS/100 watts max
*Impedance: 4 ohms

I have them wired in series (8ohms) and I'm sure the wiring is fine. My question is why am I turning up my amp so much before it reacts? I've tried three different receivers (separate receivers used to power the Aura's) and all of them need to be turned up to say 75% before the shaker start working. Is that normal?

Philips 75%
RCA Volume was at 47 out of 50
Kenwood...clip light just goes on and on and on.

Can I just hook them up as 4 ohm and let the receiver do it's thing? Will I still blow the amp even at low volumes? I'm not worried about blowing the amps just don't want to harm the bass shakers.

BTW the connection being used TO the amp is the one that would normally go to my sub so I know it's not the incoming signal either.