I set the pace of the calling + tune( I'm the bossy one so I do that ) with my melodeon. I've got a very good band who know to follow. I will speed up some dances once the dancers know what they are doing.
In several of the pubs we visited in ireland they had very unobtrusive mike hanging from an arm from the ceiling. This could be moved to whoever was leading the set. I thought that was a good way to keep everybody at the same tempo. It would also mean people with quite instruments could lead.
FloraG