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