I'm a little confused. As es345tds stated the manual clearly says "put a standard 1/4 inch guitar cable here" in the description of the Analog output. I primarily use the VDI to my Helix, but I have occasionally used the battery/analog output to play through a traditional amp.
Also, I find it interesting that the subject is about using both output at the same time and the consensus is that is bad, however you must use both when connecting to the workbench via a PC. Granted in that scenario the 1/4 cable doesn't need to plug into anything, but both are used.