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

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
  

I believe the lower the tap, the lower the output impedance and distortion, always a good thing.

Bdp, that's correct with respect to output impedance being lower on a lower impedance tap.  As a rough approximation the output impedance of a 4 ohm tap can be expected to be in area of half of the output impedance of an 8 ohm tap.  However, if the impedance of the speaker is significantly mismatched with respect to the load impedance the particular tap is nominally intended to drive, distortion will increase and power capability will decrease. 

Also, although they are in the minority there are many speakers that will sound best when driven from a higher rather than a lower output impedance.  See Atmasphere's paper on "Paradigms in Amplifier Design."

Mhwalker, I'm definitely a "he," but I appreciate your sensitivity to such matters :-)

Regards,
-- Al