Ummmm, I think I found the problem. See if you all agree?
I was using a 25 foot 1/4" to XLR cable from the back of the Roland directly into the Thump. As mentioned, I tried going from the Roland to a direct box with a 1/4 cable on both ends; then out from the direct box "thru"-1/4 fitting, to the XLR connection in the Mackie Thump, with a cable that had the 1/4 on one end and the XLR on the other. Still buzzing. So for grins, I hooked an XLR to XLR cable from the Direct Box Low-Z out, to the Mackie Thump, keeping the 1/4 plug going from the Roland to the Direct box and eliminating the "thru" connection from the direct box to the Thunp, (1/4 to xlr). NO BUZZ.
There is a definite loss of signal strength, ie: everything is coming through quieter, but I can raise the volume of the Thump to FULL, with no buzz, and I can raise the volume of the Roland Module to full, and still no buzz. But there is a definite loss of overall volume, I think because I am not using the direct box properly. Hmmmm. As I understand the Direct box, the IN and THRU are supposed to be used as I was, going from the Roland module to the IN, then from the THRU to the speaker. The Low-Z outs are supposed to carry a signal to a mixer, I suppose to use a FOH PA system, or for recording purposes. Still, I have no buzz. Now...........
In looking over the cords, I suddenly realized that the 25' 1/4" to XLR could be the problem. The 1/4" plug is STEREO! Not mono. Could this be the cause of my buzz? I'm thinking I have a stereo plug, so 2 wires are involved, yet I'm using it in a mono imput in the roland. Perhaps this is creating my ground loop?
Tomorrow I am going to get a 1/4" MONO to XLR adaptor, attach it to my XLR to XLR cord, go from the back of the Roland (1/4") to the speaker directly (XLR) and see if my buzz is still gone. I'm thinking it will be .... well, I'm hoping it will be!
Darin, I was checking out your links when I tried this other setup after reading a bit on the first link. That prompted me to try it this way. Thanks for the other thoughts as well, I may try clipping the xlr wire connectors if this doesn't work.
Thumper, I don't have the input switches on the Mackie that you mentioned, but thanks for the thoughts!