moke - it depends on the size of the hall. the more reverberant the hall, the closer the boundary between the free and diffuse fields. generally, i try to stay just inside that line, or a bit closer. since i do chamber music ensembles, i am usually at around 12-15ft out from the group. that way, if the hall is pretty live, i get a good straight sound, and if an audience is present and the hall becomes less live, i can beef it up with a bit of digi-verb during mix. if i add a pair of ambient mics out inthe hall, i will move the main pair slightly closer in (8-10ft) to get a nice intimate sound while controlling the space with the ambient mics.