Woofers getting workout, but no bass

I may be way off on this, but if your woofers are moving like crazy, you should be able to hear it. If you are not hearing the bass, maybe your listening position is in a null. I have measured a 10db difference in bass by moving my listening position back 1.5 ft.
Is it possible for a woofer to be moving like mad, but not being able to hear the "subsonic" frequencies they are creating? My woofers are behind cloth at the back of the speakers so I can't see them.

you are experiencing a very typical phenomenon as Vegasears suggests. Only a filter that eliminates those lower frequencies will eliminate this. I would venture to say that the other possible causes mentioned may play a diminuative role also. Seek out an inexpensive filter online and try it out for you to see. Then, after concluding that this may be the case, you can start getting the typical audiophile paranoia of whether this is now altering the overall signature of the music.

Rumble filter
http://www.kabusa.com/rf1.htm

Thanks for all your replies. I do think it's worse at the beginning of an LP.

My floor is concrete, but the turntable is at the back of the room where there is bass buildup. I think my stand is fairly solid, and I'm using a Terrastone platform underneath my turntable. Coincidentally, I just received this yesterday - it replaced a wooden shelf. I can tap on the terrastone shelf fairly hard and there is (seemingly) no effect on the turntable. I had the rumble issue before changing the shelf.

Dlcockrum, I have the TTWeights peripheral ring, I'll try that and see what happens.

I have to imagine this is adversely affecting the sound of LP playback...

Tried the TTWeights peripheral ring and it didn't help.

I'd be pretty bummed if I have to replace my cartridge.