Very Specific Ground Loop Question.

I have my turntable set up as follows:

Technics SL1600>Little Bear tube preamp>USB external soundcard device>PC

I am getting a ground loop buzz. The turntable ground wire is on the screw on the outlet that the turntable is plugged to. I have disconnected absolutely everything else and isolated just these connections above.

The interesting thing is that if I take the tube preamp out of the sequence, and substitute a little solid state preamp from radio shack, the buzz continues UNTIL I disconnect the ground wire from the outlet and don't connect it to anything else. The the hum goes away. The little solid state preamp uses 2 AA batteries.

However, when I have the tube preamp inline, disconnecting the ground wire from the outlet does not stop the buzz. I have also tried touching the ground wire to places all over the tube preamp to ground it there, and nothing stops the buzz.

So to summarize:

TT>tube preamp>computer (ground wire on outlet)=buzz
TT>tube preamp>computer (ground wire not connected)=buzz
TT>battery preamp>computer (ground wire on outlet)=buzz
TT>battery preamp>computer (ground wire not connected)=NO BUZZ

Can anyone help? I do not want to use the radio shack battery preamp.