Look at the specs of the receiver in the manual. It will say if it supports 6 ohm speakers.
Alternatively you can look at the back of the receiver. By the speaker terminals it will say something like 'Caution: speaker impedance 6 - 16 ohm, 8 - 16 ohm' which means you can use 6 ohm speakers for the front as long as the others are between 8 and 16 ohms.
Although they give them different model numbers, the receivers in their HTIB systems are near clones of the 5xx series and all of them dating back to the 502 have supported 6 ohm speakers.