[Meir]

Hi Giller.
Just like you said. If you connect the speakers in parallel, resistance will be 2 ohms. If you will connect them In line, it will be 8.
Just to make sure we are on the same page, full shortcut, is 0 ohms.
So the lower the number is, the resistance is higher.
If you go with 8ohms configuration per side, your unit will have less load, but you will experience decrease in volume.
At the 2 ohms configuration, you will get more volume, but the unit will work harder (after all, more resistance on the output).
The question is, what is the unit specifications, and can it hold the loade?
As far as I know, there is no head unit that can handle 2 ohms.
You will just burn the outputs.
If you are using an amp, it depends on the amp specs.
Today, most amps can handle 2 ohms with no issue.
Subwoofer amplifiers (mono) can get down to one ohm easy. So is most modern 4&5 channels amps.
So to answer your question is, go with in-line configuration.
You are going to loose some volume, but you will not blow up the unit (unless specs specefi 2 Homs operation is no problem).