I've been fooling around with some stuff, and for a specific use, I need to use an 8ohm speaker as if it were a 6ohm speaker. Unless I don't remember any of the rules, a 25ohm resistor in parallel would yield 6ohms. But how much less output would come from the speaker?
If the speaker driver is to be driven directly from an amplifer Wuffwaff is correct.
If the speaker driver is to be driven from a crossover then the resistor would be required for the crossover to give the expected bandpass. The output level well inside the crossover band pass would be unchanged.