Use 4 or 8 Ohm output tab for norminal 6 Ohm speaker?

Try them both and pick the one that sounds best.

I agree with seikosha's point. I spoke to my experienced repair tech about this and he said the exact same thing. You just have to try both to see which will work for you.

My personal experience with 6 Ohm nominal speakers is the 4 ohm tap worked and sounded the best. but my amp is a Jolida. 

Use what ever sounds better.