Remembering what I read in an interview with Carlos Santana a few years back (long before he started recording with these teeny-bopper-fanclub-magazine-group-lead-singers --hell for the last 5 years, mylocal venue LOST money by having Santana there...this year's concert is sold out already!!) ANYWAYS, Carlos used to (he may still do this) during his soundcheck, mark a few spots on the stage w/duct tape to indicae spots for his best sustain. I know that is delving into the EJ realm of voodoo, BUT we ALL know this actually is true...there are hotspots on the stage...
"This is very important...where you stand in relation to the cabinet is pretty crucial much of the time. If you have messed around with getting feedback this way then you can also get a note to sustain forever."