A telephoto or wide-angle converter does absorb a tiny amount of light Richard, maybe 1/8th of a stop, so can be ignored for all practical purposes.

You say,"I have attributed this to the camera's attempt to gain up for the darkness of the auditorium. Is this a possibility?" The answer is no, and the reason is the camera's 'ramping' (where the lens loses stops as you zoom from wide-angle to telephoto) is smoothly progressive. This means that the f/1.6 aperture is only available at maximum wide-angle, and that as soon as you creep zoom forward the lens gets slower, and more gain has to be applied to keep the exposure constant.

The only thing I can think of is this. You shoot on full auto exposure, right? Then an XCU of a well lit face will indeed use a smaller aperture than a mid shot that includes lots of darkness - hence the gain-up look.

Try the spotlight mode under these circumstances - I find it excellent on the MX300 and the VX2k.

tom.