[Early B.]

I just tried the "line in" again (from preamp to "line in" on sub amp) and I got a very loud hum, even with the preamp and sub amp turned off. No issues with the LFE.   

[Wayne1]

The probable problem is a ground loop. This is is difference in the ground potential in the house's power wiring. You are most likely using a different power outlet for the third sub than the rest of the system.

There are a few ways to eliminate the hum. First is to run an AC extension cord from the sub to the same AC outlet as the rest of the system. You could also lift the ground to the sub by using a  grounding plug adapter. Be carful about using this as the ground is there for safety.

The third way is to use a signal ground isolation transformer. There are many of these at various prices.

Blue Jeans Cable sells a couple located at the bottom of their subwoofer cable page. There are many listed on Amazon

[Early B.]

The probable problem is a ground loop. This is is difference in the ground potential in the house's power wiring. You are most likely using a different power outlet for the third sub than the rest of the system.

Yeah, the third sub is in the back of the room, so it's using a different outlet.

Just curious -- why am I getting a ground loop on the "line in" but not on the LFE?

[HAL]

The line inputs are Left and Right.  If you only use one input the other is open.  That can cause hum pickup. Try running cables for both L-R line input channels if possible.

[Early B.]

Try running cables for both L-R line input channels if possible.

That worked!  Thanks.