I wanted to be able to hook up (2) 4 ohm cabs to my 2000S. For my birthday, my wife went to an electronics guru she knows (she works in the computer/electronics field) and had a cable made up that connects the two cabs together in series so I can play thru both cabs with an 8 ohm load. I don't know how it works but according to the multimeter it does.
A series box is certainly a nice, neat way to accomplish the job, but all you really need to do is get a series connection in there somewhere. An M to F/F "Y" cable with one of the jacks flipped, or just a single daisy chain cable used between the cabs with one of the jacks reversed will do the job.
Just mark it REAL good so it never accidentally gets used in a conventional parallel wiring scheme.
And, don’t forget, as pickinatit obviously knows, the multimeter is your friend. Use it
every time before you hit that power switch; just to be sure…