Wiring at 8ohm on a headunit would be horrible. Not that it would be bad for the headunit, just that each speaker would get about 3-4 watts before the headunit started clipping.
And the headunit can't detect that something is a sub and "drop it down to 2ohm", it doesn't work like that. The headunit puts out a voltage, the speaker attached to the headunit has a certain resistance, and depending on that resistance, a certain amount of current flows. The speaker decides the resistance, because it IS the resistance, the headunit/amp has no say in the matter.
'12 Grey WRX - the daily
'05 Silver WRX STi - the toy
'01 Sentra SE - the ready-to-sell