If you are going to the trouble of using a diff amp to drive your '601s, it makes good sense to replace the stock cable with a new up-market 4 way cable anyway and use a mini XLR plug, not the big ones.
I found a new post in the Internet from a guy with "made in Austria" cup and found out that his has 4 wires. Another post claims 3 wires with "made in China" cup.