Even if it is a tube amp (granted it's not mine, but even if it were) I wouldn't be too concerned using either 4 or 8 ohms with a 6 ohm cabinet. The impedance of a speaker is "nominal", or an average range as it varies with frequency.
I can't think of the brand name at the moment, but there was one that didn't label it's impedance selector as such, nor what the impedance was at the various positions. They recommended that you use it as an additional "tone control" using whichever position you thought sounded better.