It works just fine. My T60 has a green headphone port and a red mic port. I always have an external speaker system (includes a stereo amp) connected to the headphone port. There is no other port to connect to. The sound is great, and can be made too loud for comfort. When I travel, I have a small, portable speaker set that workls just as well from the same green port.
You won't harm an external speaker/amp system with the headphone port.
Oh, and my speakers (both at home and travel) have the same stereo mini-jack that head phones do.
Jack