Speaker Impedance ... nominal 6 ohms

Agree witth Cmalak. Just be aware the 8 ohm use mmay make your amp run hotter. (than it would using the 4 ohm tap). and if you use the amp with only the 8 ohm output, it may run hotter also. So watch the temps of your amp and see if you think it is getting too hot.
(no other damage is likely, and if it does NOT get too hot, you are probably going to be fine)

Hi Kossi

Thanks for posting a thread about this topic. I always wondered about this. I always thought that if your speaker had a 6 ohm nominal impedance that either your receiver (if possible) would have to be set to 4 ohms or the 4 ohm taps on a tube amp had to be used. Good to know.

One key to this is the word....'nominal'....
If the speaker is +- a couple ohms of the rating, the test would be OK.
If there are some wacky peaks / dips, you may have some problems. Again, testing and paying attention to the amps heating will be key.

The other part of the equation is Phase Angle of the load. If the phase gets high, say 50 degrees or higher at the impedance minima, than you may also have problems....especially if such double dip occurs at frequencies which require lots of power..... Tubes generally don't like this kind of behavior.