Hi Fong,
Yes, you and Randyman are correct about series connection of speakers, but the weird thing that results with series connection is that the frequency response of the speakers then sort of tracks the combined impedance curve(s) of the two speakers with audibly bizarre and unpredictable results. Check your Circuits 1 equation for power output (volts, amperes, and ohms). But it's certainly safe for the output stage.
Randyman, you're correct about higher power output ratings at lower impedances (because there's less resistance, more current--amperes--flows, hence power in watts increases) EXCEPT for receivers that have switches for lower impedances. Those usually limit current output to protect the output stage from overheating, so actual power output in watts is sometimes dramatically reduced when the switch is in the "6 ohm" or "4 ohm" position.
In any case, your Yamaha will handle the QS8s without problems as long as you aren't trying to fill a very large room with 100 dB SPL levels.
Regards,
