You guys are right in using a stereo/mono adapter and connecting directly to MOH. One problem I see with the stereo plug is that instead of hot and ground connected, the left and right hots get connected, so what MOH receives is left-right differential, which would sound weak.

HOWEVER, I still swear by using a transformer, if for nothing else but electrical isolation; I learned this the hard way. One time I hooked up a cheap radio to an ITT 801. The radio did not use a power transformer: they simply had a dropping resistor and a rectifier diode directly on the AC power line. I hooked it up, and of course one side of my input had a path to the house current. Next thing I hear a loud bang and there's paper and foil confetti from a blown capacitor all over the KSU. Fortunately the MOH was on its own replaceable card. That's just something for you all to consider.