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.
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.