The receiver is a Sony STR-DE685, the manual doesn't says anything about
speaker impedance except in the rear view of the receiver where is a label
below the speaker connectors that says to use 8-16 Ohms speakers.



I ran a Sony @100WPC STR-DE575 running 4 ohm Cerwin-Vega RE-30's for at
least 6 months. Mostly I used them for Home theather and some music and
rarely cranked them I recently upgraded to a Yamaha RX-V630 that while
rated @80WPC is much louder than the Sony. Part of the reason for the
upgrade to the Yamaha was the 4 ohm/8 ohm switch.

I don't think the 6 ohm speaker will pose a problem so long as you don't
crank up the volume for LONG periods of time and you make sure the Sony's
air vents are kept open. If you have a situation where you do want to crank
the volume for long periods of time you could place a fan near the air vents
to keep the air in the receiver circulating.