[sdiy] Clipping summing opamp issue..

[JH.]

>If you have switching jacks on the input (and aren't using them for
>normalization), you could automatically detect when an input was in use
>and adjust the gain appropriately.  No big, expensive rotary switch...

You might as well do *passive* summing: just a resistor for each input, and a buffer stage after the resistors, but *no* virtual GND 
node, and *no* low value shunt resistor. Then, connect each of the resistors to an input jack *without* normalizing to GND. That 
way, each open input won't do anything to the signal, but each input with something plugged in will work as a shunt for the other 
inputs, and attenuate the signal. Sort of automatic gain adjustment wihout any active detection.

JH.