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By Duncan Munro (of duncanamps.com)
 
What happens when I mismatch output impedances in my tube amp?
* Lower impedance will _generally_ stress the power tubes gradually more and more as the volume goes up.
 
* Higher impedance will _generally_ stress the power tubes less as the volume goes up until the powertubes cut off at high volume levels. This will create voltage spikes on the plates of the power tubes called flyback.
 
If the flyback spikes are kept within sensible limits (say 1000V-1200V for EL84/6V6 etc., and 1500V for 6L6 type amps), then it's unlikely that any harm will be done. It's when they go way higher than this that the tubes, transformer or sockets can flash over causing expensive damage to the amplifier.
 
In summary, lower impedances _may_ stress your power tubes and shorten their life. Higher impedances are OK to a point, then it becomes Russian Roullete.
 
Where running high impedances are concerned, much depends on:
 
* Quality of output tubes * Material used for the sockets * Leakage inductance of the output transformer * Quality of insulation in the output transformer * Amount of negative feedback in the power stage * The volume you play at * The level of mismatch
 
For running low impedances, the amount of extra stress will be influenced by:
 
* The class of the amplifier (A/AB) * The volume you play at * The level of mismatch
 
Given all of the variables above, it's difficult to hand out blanket statements like X is good but Y is bad. The only totally safe advice, is to ensure that what's stamped on the cab matches what's stamped on the amp.
 
If you can't do this for practical or emergency reasons, follow LV's advice and go low instead of high - there's a lot less to go wrong that way...