The way I am thinking about transparency these days is operational and not musical: the more a component allows me to hear upstream system changes, the more transparent it is. I'm sure this definiiton is not without problems and I'll also say that transparency thus defined is not necessarily a good thing (except for a reviewer perhaps). But I think it's a good, valid definition.
By this definition, the most rransparent speakers I ever owned were Quad 57's. Thiels and Avalons were also very transparent. ProAc and Harbeth: no, sorry. Of course, I was younger when I had the Quads and I think could hear differences more easily.