There should be no problem at all using 6 ohm speakers. An ohm rating is not constant, it varies with the various frequencies. An 8 ohm speaker may actually vary from 5 ohm to 20 ohm or more.

It's really just 4ohm speakers that some amps have trouble with.

I'm reciting this from memory, if I'm incorrect I'm sure one of our more technical posters will no doubt set me straight.

getting to 2,000 posts; one year at a time vp160/qs8/qs4/ep350/m60/m2200s