Woofer pumping

Hi all,

I've seen this issue addressed before but haven't quite found a solution. The problem isn't woofer pumping from my main speakers but from the subwoofer when playing vinyl. I know the most common reason is mismatched cartridge/tonearm but I have a VPI Classic One and a Lyra Delos so I don't think that's the problem. At low to moderate volume the pumping starts,when I lower the volume and slowly raise it the pumping stops. At high volume levels the woofer cone oscillates like crazy and the rumbling is out of control. As I mentioned, the woofers of the main speakers are still, barely any hint of rumble.

It's a Rythmik sub , which I love and it has a built in rumble filter which quite frankly, on or off seems to make no difference. Could it be a placement problem? How far from the turntable and main speakers should a sub be placed?

The thought of adding an external rumble filter to the signal path kind of worries me. Any thoughts are greatly appreciated.