I don´t understand, how it works, to set the send-faders for the ER busses to "pre-fader", cause the S1 stereo imager then has (in Logic) no effect on the send-signal, meaning, that everything arriving at one of the early reflection busses is not panned.
But most people are writing, that it is best to use pre-fader-sends, cause then the main fader is controlling the dry signal, while the send is just controlling the wet signal.
So what about the panning then, if I have e.g. the brass section, playing very far away at the left? To achieve them playing far away, I need a lot of db beeing send to the ER-bus and not much direct signal.
But only my (quite silent) direct signal is influenced by the S1-imager.
IMO you should be sending pre fader (at least pre panned and full stereo --ie not narrowed) signal to the ER busses ... this is because Altiverb's stereo positioning feature itself will be giving you all the 'panning' and 'st width control' - that is exactly what moving the speakers about and making them closer or further apart does! 😊
After setting the stage positions in Altiverb I then go back to the direct signal of each instrument and set its pan to match the stage position and narrow the stereo spread to match too (virtually mono for solo instruments, wider for strings etc)
Then I set each track's (post fader) send to the Altiverb tail instance I have sitting on a buss .... the further back the instrument is placed on stage the more reverb and less direct is send and vice versa ....although in reality I just juggle everything (ER/ direct/ tail) until it sounds right!
I hope that makes sense - I am still only a novice and still experimenting with other ways! (including using the 1 Altiver with stage pos and tail per instrument or group) ......