Front speakers are 4 ohms.

Centre and rears are 6 ohms.

Does the Amp switch really need to be set to 6 ohms ? Wondering if I'll lose anything by doing so.

Thanks.

If you have a switch it should be set to match the LOWEST impedance speakers (set to 4 Ohm).

You will lose a little power driving 6 Ohm speakers from a 4 Ohm output but most likely not noticeable, even less so as these are the centre and rear drivers (if you do notice you probably need a bigger amp ).

Before doing anything - please this article, which will give you an insight into the wonderful world of impedance switches and what they do:

http://www.audioholics.com/education/amplifier-technology/impedance-selector-switch-1

I have been in the same situation as you, but ended up going with separate power amp.

Thanks - good read. Looks like I can keep the setting at 8 ohms to drive my 4 and 6 ohm speakers.

Just to confuse you even more.

The impedance of a speaker can vary wildly across the frequency range.

http://www.churchsoundcheck.com/imp1.html

http://www.deltamedia.com/resource/impedance.html