The Ohm setting limits current draw when lower Ohm speakers are connected. In effect you are preventing the receiver from delivering rated power. It is CYA by the receiver manufactures putting light weight amp section in budget receivers. Leave it at 8 since mixing 6 and 8 Ohm rated speakers is no biggie. You will need to hear for distortion in the sound at higher listening levels. It is a sign that the receiver or speaker is being over driven.
That said, nominal Ohm rating of a speaker is not a true representation of the electrical load on the receiver's amp section. Further, speaker manufacturers routinely lie about the number and give higher nominal Ohm ratings to their products.
To give you a good assessment on your situation, can you provide details on the speakers and receiver in your setup?
