I also wouldn't worry over 6 vs. 8 ohm. Shouldn't be a problem.
(BTW, the loudspeaker specs from manufacturers are often inaccurate,
and impedance will vary with frequencies also.)
They're just trying to be
more accurate in their rating. It just would not be a good idea to runs
two
pairs at once on that receiver.

Agree, unless the receiver manual states otherwise.

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.

If I was using really low-impedance speakers (such as 2 ohms), I'd put
my hand on top of the receiver and make sure it's not getting too hot
and is well ventilated, especially when the receiver is turned up to
loud volume.

So can I trust the 6 Ohm speakers will not melt my receiver as long as I
don't turn it too loud ? (Is anybody ready to put his sign under this... ;-)
?