Lee,

I do not own a Yammie, but think the 'activity' you see is caused by the 'RUNNING STATUS' communication.
Many midi-equipment is able to use only data-bytes as long as the status-bytes do not change.
Looooong time ago I owned a Korg M1 and 01W/FD and they also did a continuous transmission...... it think.
If you want to stop the activity-leds you have two options :

TX : set running-status to OFF (is possible)
or put a 'running-status filtering' in the 'receiving' machine.(if possible)

Roel