subwoofer connection

I am trying to hook up a new Pinnacle sub 10 subwoofer
to a Sunfire TGPIII, symphonic ref. amp, and Vandersteen
2CIs system, used only for music, no video.

the sub added a bit of background that wasnt there,and sounds good, but if I turn up the subs volumn above 25%,
or the frequency range cutoff above 100hz
I hear a rapid,like 200 bps heartbeat?
I can't hear with music on, but between songs or even with no source on?
I've tried different cable connections, locations, separation, and suboptions on TGPIII, but no change.

any ideas

Rmstern - does the beat occur if you have just the sub on with no connections to it? Try that...if yes, it is something with the sub. If not, then it may be something the sub is getting from the signal being fed to it (ie, the beating possibly coming from somewhere else in your audio chain).