I can't reproduce the problem. I set 5 V for channel 1 and 1 A max, connected a 10 ohm resistor with some thick test wires, 1 m long each, and I can measure 4.97 V at the resistor and 4.999-5.0 V at the terminal (measured directly at the washers on the front panel). It is configured for 230 V. Mains voltage is 226 V (true RMS, all measured with a BM257s).
I think it should work with the 120 V setting, usually there is some safety margin and then it should only show problems with higher voltages, if it is not some low-voltage tap at the transformer which is right at the edge between two ranges. 100 V should not blow it up, it just gets a bit warmer. Try to measure your mains voltage (carefully, with a good meter).
Dave did a review of the SPD3303X and there were issues with the banana plugs. The socket was slighty too wide, so some banana plugs are a bit loose in it and this caused some voltage drop, too, if you wiggle it.