want to hook up 2 spkrs for each left and right.

Um... Don't, but if you do, the safest way to run them would be in parallel. In series would drop the nominal impedance down to 4 ohms and I would imagine dips into the 2 ohm or less range which could harm your amp.

neither.

If you have a preamp out, take the signal from the preamp out to a second integrated amplifier and hook your Mythos speakers to that integrated. This way, there is no extra load on your initial amp, there will be no impedance issues at all, and BEST of all, you can control the volume of your second speakers relative to you main speakers which is ABSOLUTELY critical to the quality of the listening experience.

This is what I do exactly and the 4-speaker 2-channel set up is a monster improvement over the two speaker 2-channel set up --- but you need differential volume control to do it right... so you need a second integrated amp.

Wiring speakers in series is the safer way not parallel. If wired in series it would double the impedance.If wired parallel it would cut it in half.