I posted this in another thread, but thought I'd repeat it here.
I've found if I put them last in the ProChannel on all tracks, and the buss versions on busses, and have them switched on from the beginning works best.
I then play around with the trim and drive knobs. I do carefully gain stage to make sure none of the modules are too hot and clip. It's handy to insert an empty PC FX Chain Module in between to do this, e.g. after the Quad EQ if it boosts the signal.
If I want to hear the difference I quick group ALL the Console Emus and turn them off together. Then I hear a big difference. Turning them on and off individually doesn't. Subtle, I guess, but definitely noticeable.
The effect of them is cumulative, as Seth pointed out. It's when you hear them all on, and then all off that you notice it.
I have done this with several clients and students, and they all agree the Console Emus sound better. How they describe the change varies, but that's human nature. It's also interesting to quick group and try the S, N & A modules to compare them. I admit the SSL one is the most subtle. But I believe that is true of the SSL desks in reality, they colour the sound less than a Neve or Trident desk.
Just my penny's worth..