From the user manual:
http://download.yamaha.com/api/asset/file/?language=en&site=ca.yamaha.com&asset_id=45558
Yamaha RX-A1000
When this unit is configured for 6 ohm speakers, 4 ohm speakers can also be used as the front speakers.

I suggest trying the unit with the ohm setting at 8 and only if it shuts down then consider setting it to 6.
If you want it louder than what it can do prior to shutdown, i suggest getting a separate amp.

"Those who preach the myths of audio are ignorant of truth."