How to get really heavy treadmill downstairs?

Glad you got it down. 2 able bodied men should be sufficient. If you're stuck again, just lift it bit by bit. I wouldn't be using any carpet or anything, it will only get in the way and you don't want to be tripping over it when carrying an awkward load. If anything, wrap the treadmill in a few furniture blankets or whatever you might have to protect it and the floors etc. If you're going to go for it and can't carry the weight for the duration required, put a heavy strap around it, have the lighter side go out over the steps first while being supported by the 2nd person and then lift the heavy side step by step with the straps (saves the back bending down). Keep it supported on the step so the weight is taken and keep it horizontal enough so it won't slip down.

With items such as a piano were the weight is evenly heavy, have the stronger or 2 people ideally go down first and one above to guide.

Meanwhile, here's some impressive heavy load vs stair videos