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.

Yes, I will try the 4 Ohm tab. The problem is that I don't have the golden ears:-):-):-) Thanks for the advice.

According to this review your speakers have a nominal impedance of 8 ohms, and a minimum impedance of 6 ohms.  Also, the reviewer states that he found them to be easy to drive, and that they provided good results with an amplifier and a receiver that could be expected to not perform at their best when dealing with low impedances.

Which suggests that it is more likely than not that the 8 ohm tap (not tab) would be the one that would provide you with best results, although as the others have said trying both taps would certainly be a good idea.

Also, like many ARC amps your VS115 has an output impedance (about 1 ohm, for the 8 ohm tap, according to these measurements) that is a bit lower than average for a tube amp, which further suggests that using the 8 ohm tap is likely to be what you would find to be preferable.

Good luck.  Regards,
-- Al